I feel like I had relatively few PS2 games, but of the ones I did play, Metal Gear Solid 2 and Final Fantasy X are my favourites, and they still rank among my favourite games ever, and probably also the best games in their series (although note that I've missed a lot of Final Fantasy games despite also having played many).
I've also got to mention the Dynasty Warriors games. Of the (main) DW games on PS2, I only missed DW4, and I really enjoyed the rest. I think DW5 takes the win, although there are elements in DW3 that I liked more. Particularly the bodyguard system felt more immersive in DW2 and DW3 than it did in DW5.