I didn't play that many PS2 RPGs. The ones that really made an impression on me:
Star Ocean 3 (yeah the ending was pretty questionable but as Mirage pointed out the gameplay in this game is excellent, plus it has one of the best soundtracks on the PS2)
Shadow Hearts
Final Fantasy X (it has some rather glaring weaknesses but I enjoyed it fairly well regardless)
Dragon Quest VIII
Not sure how I'd rank them. There are also a lot of important games that I missed or didn't play enough to form an opinion about, such as FFXII, Suikoden III-V, Xenosaga, SMT/Persona series, etc.