Based on number of replays: Deus Ex
Based on number of hours: Tribes
Based on number of hours in a family of games: HL, Counterstrike, TF2
H.
For me it’s a toss-up between Civilization, Master of Orion 2, Star Control 2, Privateer, System Shock 2, Fallout, Thief, Ico, Rez HD, Psychonauts, Super Mario 64 and Fable 2.
Gosh, I guess I have too many favorites across too many genres. Oh well. I can live with it.
Other: Zelda
Especially Zelda 2, 3 and the wind waker.
Updated the ‘Other’ votes. If you selected other and voted multiple times I haven’t counted your votes yet :
Houngan
Thief
There are some great games up there, but OOTP is still my favorite.
What is OOTP?
It was really hard to choose between X-Com and Planescape, but Planescape it is.
Other: Dark Age of Camelot
I can’t believe I’m the first person to vote Diablo/Diablo2 after nearly one hundred votes. That game (esp. the first) owned me for years. It is the veritable Obsidian RPG of the Zodiac.
Honorable mention to Civ (just started playing IV recently for the first time; still as much ‘just one more turn’ fun as ever), to RockBand and to Dark Age of Camelot.
Fallout series, with 2 being my favourite (I loved all three though).
i think half life is the game that i return to most often in its various incarnations so probably that.
Okay, that’s good news. I almost put you down for Harry Potter: Order Of The Phoenix ;)
This may not be the best game I’ve ever played, but it had the best effect on me of any game ever, and is therefore my favorite:
Wolfenstein 3D
The Nazis in that game were the best Nazi’s I’ve killed – human enough to be Nazis and therefore fun to kill, but inhuman enough so that killing them evoked no empathy. And (spoiler) killing Hitler was the best gaming experience I’ve ever had and could ever imagine having.
Il-2 Sturmovik. And before that another flying shooting bombing game with simulationist tendencies.
Other: Fallout
Bases on replays: Deus Ex
Based on “oh what can I break this time oh shit 3 hours are gone”: Grand Theft Auto
I honestly can’t say one over the other, but let’s say Deus Ex if only because my GTA love is spread over 6 iterations now.
Half-Life.
Honorable mentions go to Rock Band and Grim Fandango.
Zelda. sorry