Right now it's Ty the Tasmanian Tiger. I haven't played it in a while, but it's a charming enough 3D collect-a-thon.

Though, for all time favorite, nothing beats two of the only first person shooters I play anymore: Star Wars Battlefront 1&2. Basic answer, but it's two games I still occasionally play to this day, unlike most every other PS2 game I had growing up.