Marathon trilogy
which later led to UT (yes, the best of the series for gameplay, and I still play)
ROBOSPORT
Prince of Persia
Monster Fair Pinball (best Mac pinball game ever, maybe best computer pinball game ever)
Flatout2
Of those, Marathon was the game that took me in, felt like I was that character, and tons of hours playing on lan, so that's my No. 1
Flatout2 I discovered just a few weeks ago - something about flying down side streets at insane speeds, doing a powerslide through the plate glass on the corner of a building and into an opponent to knock them into a wall, then hitting the nitro down the straightaway makes me giddy. heh Like an updated Carmageddon, with better everything.
Doesn't seem to be many if any other macheads playing it though. Runs great on 2 ghz intel imac with ATI 2400