I'll reply using genres:
Action-FPS: Half-ilfe: I bought this game over Sin [remember that one] and was totally blown away. The graphics were good but the entire gameplay was so emerssive it just sucked you in until the end of the game. I found the puzzles to be interesting but the 'scripted' scenarios were the best thing, every where you went you were challenged with something almost unique.
RPG: Daggerfall: Bug ridden and waaaaaaaaay too long, this game had so much to offer. The randomized quests and dungeons were cool and the way you could build up your character was great. Creating your own spells pwned.
Strategy: Master of Orion 1: The other games in the series were sub-par but the first one remains as a classic in my book. Not too overwhelming in it's complexity, this game was fast to learn and was entertaining as well. Playing the Darloks was awesome...
Survival-horror: System Shock 2: I know it's a FPS-RPG, but I just had to create a genre so I could mention it. This game was truly unique for me. I was hooked from the opening moment. The re-spawning creatures made you never feel comfortable and the weaknesses of your character chioce always made you think about what your next step would be. You didn't dare just run into a hallway or room without taking a sneak peak first.
Online: Diablo 2: LOD: I don't have a lot of time to play an online game so I want a simple, hack 'em up, but one that'll last for a while. I found D2 to be exactly what I wanted. The games could be quick or take all night. The items make the game, and playing it to collect better items makes the game for me. I could care less about an in-depth, complicated game for online. Just make it simple and fun.
Zconnor