Most of my choices have already been listed, but I have one which never seems to come up: Urban Chaos: Riot Response. If you're looking for a first person shooter it's the best one I ever played on PS2. Basic storyline, you play a Riot cop in a city that's gone to hell because of a gang of arsonists, aptly named the Burners. You have access to a pretty decent arsenal, plus an upgradeable taser and a riot shield which can be used to deflect bullets and also bash people's heads in. There's also some decent gameplay to be had in directing allies such as firemen and EMTs to rescue civilians, but the saving grace was the upgrade system. It reminded me of GoldenEye quite a bit, you had to complete certain objectives within a level to upgrade weapons, magazine capacity, armor, the taser, etc... The tasks usually centered around completing an objective in a time frame, taking out so many people with head shots, using non-lethal takedowns on so many people. So it required some skill to get better, because it wasn't always easy to take down a gang leader in a nonlethal manner, but it was still possible and very satisfying when you did it.
It came out rather late in the life of the PS2, 2006, and the only reason I found out about it was because there was a demo for it included with a copy of Hitman: Blood Money that I rented. Still a great game, and I'd love to see a sequel.
