Either Persona 4, kingdom Hearts, or Wild Arms: Alter Code F but needless to say there were a lot of fantastic RPGS for the PS2!
Valkyrie Profile 2, Way of the Samurai 4, or XenoSaga
For me it's Persona 3, Persona 4, Shadow Hearts Covenant, Final Fantasy X, and Xenosaga 3. Even though I did not list it, Star ocean 3 till the end of time will always hold a special place in my heart, since it was the first RPG I ever got into. Good memories with that game!
Grandia 3 and the suikoden series
Radiata Stories is one of my favorites, although I haven't met many people who have played it.
If I had to pick just one I'd probably go with Final Fantasy XII, just because I'm such a Yasumi Matsuno fan (Vagrant Story being my favorite game of all time).
Kingdom Hearts II
Final Fantasy X
Dragon Quest VIII
Xenosaga: Episode I
Valkyrie Profile II. It has really nice combat, and it's one of the like 5 1080i ps2 games so it looks hella crisp