I'd imagine that the original Unreal Tournament demo is high up there, 2003 and 2004 probably got played a decent amount too. Does Half-Life: Uplink count? Cause I'm pretty sure I replayed that one a fair bit too.
Outside of those, I'm not too sure! I used to buy and subscribe to several PC gaming magazines that came with demo discs on them, so I played a looooot of demos. First one that comes to mind when I try to think is the Hulk game from 2003, so that'd probably be one?
I didn't have a Playstation myself, but I do remember that I played at least the Crash and Spyro demos over at my friend's place a loooot of times.
...maaaan, I really miss Demo Derby.