I can't say "changed my life" like I had some big epiphany, but they definitely changed how I look at games.
Fallout 2: My first Western RPG, and thereafter I had a hard time playing any RPG with bad or disconnecting storyline or that fell apart towards the end; I.E pretty much every JRPG except Chrono Trigger and Earthbound, and many western RPGs too.
Unreal: One of the earlier shooters, and a fantastic one. Mix of sweet and intelligent enemies, fantastic level design, and innovative-without-being-useless weapons. set the bar higher for FPS
System Shock 2 and Deus Ex: I'm lumping these together because they're along the same lines (and I don't remember which I played first) that show how to do a FPS/RPG game, and do it well. I've seen a couple good attempts since then, but have yet to see a game that truly shines like these two in the combined genre
Far Cry: I saw what FPS games could do when they allowed a good mix of outdoor exploration, sneaking, and a fucking brilliant blend of short and long range combat. I've never seen another game that did those things so well (including Crysis), and I've come to expect it anymore. Thus, I've been pretty disappointed in newer FPS
City of Heroes/Villains: Most fun I've ever had with an MMO. Good storytelling, a solid attempt to break up grind or at least make it feel less grind-y, best character design I've ever seen, and the longest-running comic book universe I've seen that never needed ret-conning or fall into the batshit insane category. I never tried AoC or WoW because of it, feeling simply "I'm having fun now, why should I care about them?"
Doom 3, Crysis, Jericho: I mention these for a simple reason: I've pretty much abandoned all hope for the FPS genre at this point. I doubt I'll buy another such game for a long time.
Oh yes. I guess I should mention Planescape: Torment. One of the first games that really made me *think* while I was going through it, about morals and philosophy, not just how to win the game.