Never had my own PS2, but my friends and roomate's PS2's got plenty of play in college. Top games were:
Gran Turismo 3 (spent an entire semester on that, don't remember what classes I was taking)
FFX and FFX-2
Madden 2004/NCAA 2004 (not PS2 exclusives, no, but got the most play)
Oh yeah, Tekken Tag, hah, I killed on that one with Eddy (who didn't?). What else, GTA3, did anyone else ever play Ephemeral Fantasia? Crazy-strange game that one.