List is in no order:
1) Odin Sphere-
Beautiful graphics and animation, unlike anything else out there (unless you include the JP Princess Crown)
2) Wild Arms 4-
Not the best on PS2 by any means, but the characters, story, and gameplay, as well as music are all decent enough to give it a try.
3) Persona 3-
I tried very hard to get into this one, I saw the appeal but honestly some things put me off (god awful battle music, party members are AI controlled, other stuff) but there are some people that swear by this game, so check it out!
4) Rogue Galaxy-
Great graphics, nice game world, but you gotta be into Dark Cloud-esque gameplay or to put it as action RPG combat.
5) Baulder's Gate DA-
I never played the second, but if the first is an indication it would be very addicting and a fantastic Diablo type loot whoring dungeon crawler, great graphics too.
6) Final Fantasy XII-
This is a love or hate for FF fans, basicaly becuase of the combat system. It mirror's FFXI's combat which I was very used to since I had over 4000 hours in that game. I really liked everything about 12, especialy the combat which made battles go super quick.
7) Romancing SaGa-
Not the best out there at all, infact alot of people hate it. But you might like the games awkward character designs and beautiful environments. Definitly try it if you can find this super cheap.

Unlimited SaGa-
Probably best to avoid this one by all means, though the battles arent half bad everything else about the game is pretty terrible.
9) Star Ocean TTEOT-
Huge game, too long for me to finish but the combat is great for sure.
10) Dark Cloud-
One of my favorite PS2 games ever (I used to run a popular fan site for it during the games launch) leveling up my weapons was so addictive, and building towns was great. The game has not aged well though, but its worth a try.
11) Dark Cloud 2-
Great graphics and greatly improves on DC1's gameplay.
12) Kingdom Hearts-
Worth playing even if you think you will hate the game, believe me it is a great mix of two worlds.
13) Final Fantasy X-
Not my favorite FF by any means, but it does alot of things right.
14) Dragon Quest XIII-
One of the best RPG's to grace the PS2, it takes a classic formula and makes it feel fresh by adding memorable characters, fantastic classic RPG gameplay, and beautiful graphics. If you love RPGs you cannot pass this one up!
15) Xenosaga-
It will appeal to you if you like long CS's and getting to know your characters and see them develop. The gameplay itself isnt bad just ALOT of long cut scenes.
16) OKAGE Shadow King-
An overlooked gem, its a traditional turn based RPG with a "nightmare before christmas" art style. Extremely quirky really nothing else like it on PS2. Try it out.
17) Grandia III-
I still stick by Grandia III as having the best combat system for any RPG I have ever played. Being able to juggle enemies and do all sorts of combos with good timing in a semi turn based semi action rpg combat system. It also has great graphics and a pretty good character cast. One of my top 5 for PS2 RPG's.
18) Ephemeral Fantasia-
The first traditional RPG on ps2. A total mediocre game, with a time changing element that determines what NPCs and characters you may find to recruit. Very bare bones but is worth it as you can find it for like $5 used.
19) Legaia 2-
Didnt play too much of this one, but I definitly think it has one of the better RPG combat systems out there with different button presses for attacks. Keeps things from getting monotonous.
20) Orphan-
One of the launch PS2 games. Its also not as bad as many think, I found the combat to not be half bad...but be prepared for terrible terrible voice acting.
21) Summoner-
One of my personal favorites, though it has not aged well at all, I really enjoyed tearing up monsters with my party of AI controlled partners. The game also felt quite large thanks to large open environments.
theres more out there but these are just a few for you to check out. Also games that are not RPG but you need to play them anyway:
Ico
Shadow of the Colossus
Beyond good and evil
Klonoa 2
Rez