In no particular order:
*
Half-Life I & II - actually, these are probably no. 1 (1997-2004) (PC)
* The
Quakes I-III (not played QIV) (1996-1999) (PC)
*
Doom II (1994) (PC)
*
Lemmings (1991) (Atari ST, motherfuckers!

)
*
Worms (1994-1995) (the original, plus
Reinforcements - not the sequels) (PC)
*
Xenon II (1989) (Atari ST)
*
Unreal Tournament (1999) (PC)
*
Repton III (1986) (erm, Acorn Electron...)
*
Descent I & II (1995-1996) (PC)
*
Gauntlet II (1986) (Atari ST)
Edit: plus the usual obligatory classics, Street Fighter II, Tetris, Space Invaders...was never into Mario or Sonic, though. Oh, and Llamatron fucking rocked!
