I want to say Shadow of the Colossus for all of the advanced techniques it was using (faked HDR lighting, deforming collision detection, inverse kinematics, fur shaders on giant colossus models, self shadowing... etc.), but I think the overall best in all areas has to be Silent Hill 3. Even today it manages to look polished, modern, and artistically sophisticated. Its cohesive look stands the test of time better than any other PS2 game, IMO.
PS - I'm also on a PS2 kick right now, OP. Best console of all time.
PS - I'm also on a PS2 kick right now, OP. Best console of all time.