1 street fighter alpha 3
2 dragon quest 5
3 start ocean 2 (the original)
4 Shadow of the Colossus
5 godhand
6 final fantasy vi
7 shin megami tensei III: nocturne
8 devil may cry
9 darkstalkers 3
10 gothic II
FPS : Halo and Left4Dead
RPG : Fable, the Lost Chapters
Action 3rd Person : Batman Arkham Asylum, Assassin Creed
Horror : Silent Hill Series (just the first 3)
Strategy : Brigandine , KKND, and Fire Emblem Series
Depends on the genre, but for RPG's it would have to be Neverwinter Nights (I can't believe no one mentioned these or Baldur's Gate! ;O ) tightly followed by the Mass Effect series now. For MMO's probably Ragnarok Online, (Not that the game is overwhelmingly good, but the community of it is.)
For Hack n' Slash or action games nothing beats Ninja Gaiden for the Xbox (Well, maybe Sigma for the PS3... maybe)
Otherwise... Well.. WarCraft III never grows old... ^^
The ones I always come back to are Morrowind (with Expansions and Mods) and Starcraft: Brood Wars. Something just keeps drawing me back to them.
Right now though, I'll go with Super Robot Taisen OG Saga: Endless Frontier. Jiggly Boobs, Guns, Swords, Mechs, and Explosions, don't make a great game but boy do they help.