Some say the mere mention of deus ex makes at least one person reinstall it. What are the games you go back to again and again no matter how old? I bought a game called Defense Grid the Awakening. I play it all the time. What's your "always" game?
There are many always games, but the one that seems to get the most plays for me is Worms Armageddon especially now that I have it on PS3 with online play.
Bubble Bobble, Gunstar Heroes, Bare Knuckle II, Metal Slug (and 2 and X), Gears(and 2). My desert island games if you like.
I keep going back to Zelda: LttP and Doom (the first one). Worms (first one, again), Battle Isle II and quite a few adventure games (Lucas Arts, Sierra, Simon the Sorcerer series) get a lot of playtime too (despite adventure games having notoriously bad replay value). Then there's some games I never finished, and probably never will, like I Wanna Be The Guy, but I just keep trying nevertheless... Lately I'm playing a lot of Galcon Fusion. Awesome game, simple but really addictive, can't see me getting tired of it any time soon.
Gunstar Heroes, the first Viewtiful Joe, MD Sonic games except for 3D, a lot of 2D shooters, especially Saturn ones...I also revisit some of the GTA games, mainly IV and San Andreas, because they're fun without any set objectives. Lucasarts adventures are also ones I like to play over again, who cares if I've memorized what I need to do, the writing's just too good.
Quake, Umihara Kawase/Shun, Diablo 2, Tetris, Super Mario Bros, Sonic 2. Can't play Quake or Diablo 2 any more due to RSI
Final Fantasy VIII, Grand Theft Auto: Vice City, Super Mario Bros. 1-3 and World, Sonic 1-3K, Metal Slug, Marvel vs. Capcom 2, Street Fighter Alpha 3, and Tony Hawk's Pro Skater 2 come to mind.
Ocarina of Time, Link's Awakening DX, SOR 1&2, Goldeneye, Smash Bros Melee, Age of Empires 2, Final Fantasy Tactics Advance, any Pokémon game, ...
Earthsiege 2. Problem is the further and further I get away from a Pentium 133mhz with 16mb of ram and a Sound Blaster 16 the further I get from it being 100% playable with the trimings. Console wise I love myself some Vectorman.