Mine have pretty much all been said before: X-Com (the first one only), Fallout 2, Planescape Torment, Baldur's Gate 2 (without expansion), Diablo 2 (with expansion), Divine Divinity, Arcanum, Jagged Alliance 2, Incubation, Nox,...
But that's just Isometric RPG's.
Other good games for me are C&C Red Alert, Warcraft3, Hitman2, Dungeon Keeper, Deus Ex, Unreal Tournament, Age of Empires series, Mechwarrior (preferably the 2&4: Mercenaries), System Shock (1&2), Thief (1&2), Morrowind, Jedi Knight2: Jedi Outcast, Splinter Cell, GTA series, Battlefield1942.
This includes a lot of recent games, because I forgot about a lot of older yet fantastic games. Thing is, I could make a list of my favourite games, but I'd be leaving too many good games out.
