No order, apart from genre:
FPS:- Doom 1/2 (especially with the mod/map community that built up. Even made a few maps myself. And AliensTC was the best total conversion I've ever seen of any game then, and now)
- quake 1 (especially threewave CTF)... the reason most of (oldies) are here at nzg.com (R.I.P quake.gen.nz). And the start of the organised clan matches for me... <3 Hurricane....
- Unreal (SP) &/or UT2k3 (MP)
- DN3D - shake it, baby
- BF1942 (including DC) - so many hours, and a return to the camaraderie of Q1 days
RTS:- SC/BW - simply the best RTS ever
(Click 'n' point) Adventure Games:- Day of the Tentacle (I still consider this superior to the MI series ... just)
- The Dig - how many fucking times can you use a shovel?
- Simon the Sorcerer - was nice to have hand-painted scenes, British humour, and reasonably logical puzzles
Arcade:- Supaplex (Boulder Dash clone), still got a support community around (at least it did a few years back)
- Superbombliss (Tetris clone)... sunk many a gold coin playing multiplayer into this bad boy as a uni student
ok, might be slightly more than 10.... but fuck ya!