You know, after a bit of replaying the ratchet and clank series reminded me why it took up so much of my childhood. I just love the combat and characters in it so much, it's fast paced challenging and most importantly fun. I may put R&C 1-3 on my favorite games list now. I can't put just one up there, they are all great for different reasons, each with it's own flaws and pro's that make it unique.
Jak and Daxter, Jak II, and Jak III were also fantastic plat-formers/shooters. The Sly Cooper series also remains as one of the ps2's best series. I already mentioned Okami, but Ico and Shadow of the Colossus also need to be added. Those 3 games were just art to me, beautiful games. Devil may cry, Kingdom hearts, FF 10/12, I just can't do favorites.
The Ps2 is obviously my favorite gaming system for obvious reasons.