Delta Force: Black Hawk DOwn

1. Dragon Age: Origins - This game just does everything.  Simple but engaging story that benefits greatly from the character development, which is some of the best in any game.  The plot and character development are more branching and more dynamic (responsive to player interaction) than any other game.  Solid party-based RPG gameplay offering a decent amount of playstyles while being very smooth.  The combat tactics customization is amazing.  Six different origins (90-180 minute playable openings) based on your character build.  It takes no less than six playthroughs to see everything it has to offer, and each one will last 50+ hours excluding the expansions.  It has two fantastic expansions in addition: Awakening and Witch Hunt.  I currently have 297 hours in it: four completions of the campaign, two completions of Awakening and Witch Hunt.  The main campaign has yet to get repetitive due to how diverse it is.

2. Mass Effect trilogy - For similar reasons as the above game.  I beat the first game twice, so that's 55-60 hours.  I finished the second game and all story DLC twice as well, in 108 hours.  I only finished ME3 once (which included all story DLC), since I'm playing lots of new games at the moment.  I group this as one game since Mass Effect is really one game split into three parts.
 
3. The Elder Scrolls V: Skyrim - The most content rich, long lasting game there is.  Along with Fallout 3, this game has by far the best and most detailed open world design.  Complete freedom and very unhinged role playing.  I have 749 hours across 7-9 characters (lost count and playtime is far from equal on them).  I still haven't done everything the game has to offer, and I've only finished the campaign once.
 
4. Fallout: New Vegas - Although the open world design/atmosphere is a bit more bland than FO3 (mods really help though), everything else is significantly better.  It trades Bethesda's superior open world design for Obsidian's vastly superior writing, which is some of the best there is in the video game industry.  It has some of the best, most complex characters of any game, and such extensive role-playing.  279 hours currently, with two completed characters and one WIP Legion character.

5. Fallout 3 - For similar reasons compared to Skyrim, though it's not quite as content rich.  200 hours currently, across 3 characters.
 
6. Neverwinter Nights 2 - So much content, so much role-playing, so much gameplay variety.  70+ playstyles when combining the amount of classes, sub-classes, and prestige classes it offers.  Combined with the hundreds of spells it has across 8 different schools of magic, and the amount of custom campaigns available, it never gets old.  Its expansions really elevated its spot on my list: each one has very unique gameplay, and so does the base game with its micromanagement that inspired Dragon Age: Awakening.  Mask of the Betrayer has some of the best writing of any game, the closest thing to Planescape: Torment.  Storm of Zehir has very distinct micromanagement and a highly interactive world.  I can only estimate my playtime, but I estimate it to be around 150 hours: one playthrough of the main game (which I estimate to be around 60 hours), one playthrough of MotB (around 30 hours give or take a few), one WIP playthrough of SoZ (I probably have around 40 hours in it), and one playthrough of Icewind Dale - NWN 2 mod (which took 20-30 hours).
 
7. Deus Ex: Human Revolution (Director's Cut) - Amazingly detailed world, good writing, intriguing story, diverse and excellent gameplay both for stealth and action approaches.  It took about 50 hours to beat the original + Missing Link.   
 
8. The Elder Scrolls III: Morrowind - The unparalleled, visionary, and unique art design did it for me.  Such a vast, alien open world, that's still amazingly detailed by today's standards (only bested by newer Bethesda games in how detailed and living the world is).  It's also a very comprehensive RPG.  Only 38 hrs so far, all I've done is explore and do a few quests.
 
9. Star Wars: Knights of the Old Republic - I love Star Wars and I love RPGs, so obviously I was bound to love this game.  So far only one playthrough (47 hours) but I can go back in and play another at any time.

10. Star Wars: Knights of the Old Republic II - The Sith Lords - Although it was rushed which resulted in plot holes that hurt how well the game flows, the gameplay has improved over the original and the story is actually unique and more complex than what we're used to getting from the Star Wars franchise.  So far only one playthrough (60 hours) but I can easily replay it at any time.  I'm just holding off and playing other games instead.
 
The only ones I don't use mods for are the Mass Effect trilogy and Deus Ex HR. 

A DOS Game called Midway, written in MS Basic.

The games I've replayed more than twice, in no particular order:
 
Thief 1-3
System Shock 2
Deus Ex
TES: Morrowind
Monkey Island 1-3
The Longest Journey
Syberia 1 & 2
Gothic 1 & 2
Hitman: Blood Money
Mafia
 
Of course there are others, but they've been mentioned already (Mass Effect, KOTOR, etc).

 Lot of great games listed here already.
For me system shock 2 still can give me chills.
boredgunners list also fits my type of games very well.
Neverwinter Nights is a favorite of mine.
And for some really retro gaming Zork the underground Empire.

Well there are just so many...
 
Xbox(Original):
Both SWTORS.
Star wars Battlefront 1&2.
Grand theft auto: San Andreas.
 
Gamecube:
Metroid prime 1&2. Played them both at least 45+ times each.
Star wars Jedi night, Jedi outcast and Jedi Academy.
Star wars rouge squadron (all of them)
Star wars pod racer, can't remember the name, but I played hours and hours and hours of it.
 
Xbox360:
COD4.
BF:BC2.
COD Zombies (Only the original with Dempsey, Nikolai, Richthofen and Takeo).
Fallout 3.
L4D1&2.
Burnout paradise.
A lot more I won't keep going on and on.
 
PC:
Fallout 3
Fallout:NV
CS:S
All 3 Dead spaces.
TESV:Skyrim.
Killing floor.
Crysis 2&3 (never played 1).
TF2.
 
I know the list seems short with my PC but there are other games that I have stopped playing simply because the developers got greedy and destroyed the game with pay-to-win weapons, etc etc. (Runescape, AVA, Soldier front, Warframe, etc etc).
 
 

Rust (Experimental): It is nearing baseline playability meaning it is nearing the playability of the vanilla discontinued Rust. Updates are provided on a daily basis. It is in the alpha stage but highly addictive and very playable even now. The development team is very innovative working closely with its players.

It is another survival game but is bar none the best of its kind. Zombies were in baseline, and not in Experimental - initially zombies were designed as a placeholder. The game is built using the Unity engine (version 5 beta) and it's looking gorgeous.

It is only a multiplayer game, multiplatform and capable of up to 500 players on dedicated servers. No two maps are the same as they are procedurally generated using 'seeds.'

I highly recommend this game!

My favorite game is probably Xenogears.  Of course, this comes after the long line of other Square/Enix games of FF, FFIV, FFVI, Dragon Quest I, IV, etc.
 
Best game is easily Chrono Trigger.  It's about as close to perfect as any game can get.
 
On PC:  CoD 4:MW, Assassins Creed series, Tomb Raider (2013), Saints Row IV, Portal/Portal 2

Novalogic- Delta Force LandWarrior
Fun addictive game, but it was certainly the group of players 1999-2001 and the hardware performance closeness that made it really special.
A amazing experience that will never happen again. Certainly the finest gaming period of my life bar none

Quake 3 Arena
Soldiers of Fortune 2
BF2
F.E.A.R. Combat
SupCom: Forged Alliance
Mirrors Edge

That reminds me of a favorite of mine I forgot to mention:
 
Tom Clancy's Ghost Recon (the original)
 
What a fantastic tactical shooter! I remember playing countless hours of 3-4 player multiplayer and having a blast.

boredgunner
1. Dragon Age: Origins - This game just does everything.  Simple but engaging story that benefits greatly from the character development, which is some of the best in any game.  The plot and character development are more branching and more dynamic (responsive to player interaction) than any other game.  Solid party-based RPG gameplay offering a decent amount of playstyles while being very smooth.  The combat tactics customization is amazing.  Six different origins (90-180 minute playable openings) based on your character build.  It takes no less than six playthroughs to see everything it has to offer, and each one will last 50+ hours excluding the expansions.  It has two fantastic expansions in addition: Awakening and Witch Hunt.  I currently have 297 hours in it: four completions of the campaign, two completions of Awakening and Witch Hunt.  The main campaign has yet to get repetitive due to how diverse it is.

2. Mass Effect trilogy - For similar reasons as the above game.  I beat the first game twice, so that's 55-60 hours.  I finished the second game and all story DLC twice as well, in 108 hours.  I only finished ME3 once (which included all story DLC), since I'm playing lots of new games at the moment.  I group this as one game since Mass Effect is really one game split into three parts.
 
3. The Elder Scrolls V: Skyrim - The most content rich, long lasting game there is.  Along with Fallout 3, this game has by far the best and most detailed open world design.  Complete freedom and very unhinged role playing.  I have 749 hours across 7-9 characters (lost count and playtime is far from equal on them).  I still haven't done everything the game has to offer, and I've only finished the campaign once.
 
4. Fallout: New Vegas - Although the open world design/atmosphere is a bit more bland than FO3 (mods really help though), everything else is significantly better.  It trades Bethesda's superior open world design for Obsidian's vastly superior writing, which is some of the best there is in the video game industry.  It has some of the best, most complex characters of any game, and such extensive role-playing.  279 hours currently, with two completed characters and one WIP Legion character.

5. Fallout 3 - For similar reasons compared to Skyrim, though it's not quite as content rich.  200 hours currently, across 3 characters.
 
6. Neverwinter Nights 2 - So much content, so much role-playing, so much gameplay variety.  70+ playstyles when combining the amount of classes, sub-classes, and prestige classes it offers.  Combined with the hundreds of spells it has across 8 different schools of magic, and the amount of custom campaigns available, it never gets old.  Its expansions really elevated its spot on my list: each one has very unique gameplay, and so does the base game with its micromanagement that inspired Dragon Age: Awakening.  Mask of the Betrayer has some of the best writing of any game, the closest thing to Planescape: Torment.  Storm of Zehir has very distinct micromanagement and a highly interactive world.  I can only estimate my playtime, but I estimate it to be around 150 hours: one playthrough of the main game (which I estimate to be around 60 hours), one playthrough of MotB (around 30 hours give or take a few), one WIP playthrough of SoZ (I probably have around 40 hours in it), and one playthrough of Icewind Dale - NWN 2 mod (which took 20-30 hours).
 
7. Deus Ex: Human Revolution (Director's Cut) - Amazingly detailed world, good writing, intriguing story, diverse and excellent gameplay both for stealth and action approaches.  It took about 50 hours to beat the original + Missing Link.   
 
8. The Elder Scrolls III: Morrowind - The unparalleled, visionary, and unique art design did it for me.  Such a vast, alien open world, that's still amazingly detailed by today's standards (only bested by newer Bethesda games in how detailed and living the world is).  It's also a very comprehensive RPG.  Only 38 hrs so far, all I've done is explore and do a few quests.
 
9. Star Wars: Knights of the Old Republic - I love Star Wars and I love RPGs, so obviously I was bound to love this game.  So far only one playthrough (47 hours) but I can go back in and play another at any time.

10. Star Wars: Knights of the Old Republic II - The Sith Lords - Although it was rushed which resulted in plot holes that hurt how well the game flows, the gameplay has improved over the original and the story is actually unique and more complex than what we're used to getting from the Star Wars franchise.  So far only one playthrough (60 hours) but I can easily replay it at any time.  I'm just holding off and playing other games instead.
 
The only ones I don't use mods for are the Mass Effect trilogy and Deus Ex HR. 

I've never said it bro... but almost every single post I've ever seen you write, I've completely agreed with; no bro-mance here, just sayin'. BTW I dig NWN as well, but haven't been back to it in years. Skyrim is also a fetish of mine, but for some reason lately, I've felt like the world was just a little... dead? I wanted more out of the dwarven dungeons and underground labyrinths. It's still an epic game for the ages. Can't wait for the next chapter in this series...

You guys are sad.
 
Mine:
Enemy Territory( game I professionally competed in)
All the DOOMS
All the Quakes especially Q3
And of course, Wolfenstein 3D.

I usually play through a game to the ending and never play it again. If it is really good, I will go back and try to complete everything 100% but that's rare. I can count on 1 hand the games that I replay for PC and it's usually because a sequel is coming and I want to invoke the mood. The Metro games, Half life games, Portal games, Batman games and the new Wolfenstein because of the two timelines.
 
Funny because If they were in front of me, I would probably play some Nintendo games all over again that I beat the crap out of like MT Punch out, Metal gear, Contra, Zelda, Kid Icarus ect ect ect.

Nice to see Thief Gold get some love.  I didn't like the last few missions though, too much running and being chased.  The rest of it was outstanding however.
 

agent8
I usually play through a game to the ending and never play it again. If it is really good, I will go back and try to complete everything 100% but that's rare. 

Some games can be vastly different when replaying them, namely RPGs such as the ones I listed.  The plot and dialogue can change completely, and so can your relationship with characters in the game (Bioware RPGs do this the most).  Plus they offer a variety of very different gameplay styles.  If you haven't experienced this, you'll be in for a pleasant surprise.