I didn't much like KotOR, the second was bit better.
Couple other nice ones I've played are Unreal II and Myst's interesting spinoff Uru.
Ones I have tough time choosing between:
Max Payne 2: but I did like the first more and I already voted that.
XIII: visually stunning and there are lots of cool and interesting elements, but occasionally bit clumsy gameplay.
Beyond Good & Evil: beautiful game, but I didn't like the ending, in number of ways.
So that leaves me to go with Rachet & Clank: Going Commando; the second best PS2 game I've played.