PS2 by the most ridiculous margin in the world. Seriously, PS2 destroyed the competition in RPGs (Japanese ones), fighters, music/dance type games, platformers, niche Japanese games, story driven action games (like Onimusha, Zone of the Enders, etc. type), etc. I have 199 PS2 games vs. 41 Xbox games and 28 GC games. I only see the margin getting more ridiculous. Also, I found Sony's first party games to be the best during the generation.
After PS2, it would probably be Dreamcast, due to having an excellent mix of games, and a lot of memorable games. The problem is that its life was too short.
Xbox had some great games I liked, but really struggled in my favorite genre (JRPGs and platformers to an extent).
Gamecube had a few great games, but it struggled in a bunch of genres I like (especially fighters and music/dance type games).