I copied the most updated list I could find of current PS2 Rpg's. I listed the ones I played and what I would rate them, I deleted a few games like Harvest Moon which weren't really rpg's imho. I'm not into strategy games or what I call faux RPG's i.e. X-men games so I haven't played most of those:
.hack//G.U. Vol. 1//Rebirth. Didn't play, I got sick of the original .hack
.hack//G.U. Vol. 2//Reminisce.* Didn't play, I got sick of the original .hack
.hack//G.U. Vol. 3//Redemption.* Didn't play, I got sick of the original .hack
.hack Part 1: Infection. Played, was decent and good for a first time game 8/10
.hack Part 2: Mutation. Played, just like part 1, continued the story 7/10
.hack Part 3: Outbreak. Played, this is where it starts to get tedious, battles are longer but not more fun. Not hard, just boring and repetitive. 6/10
.hack Part 4: Quarantine. Didn't play, currently trying to sell it, have no intention of playing through more tedium. Watched the anime already.
Ar tonelico: Melody of Elemia.* Haven't played.
Arc the Lad: End of Darkness. Haven't played.
Arc the Lad: Twilight of the Spirits. Haven't played.
Atelier Iris: Eternal Mana.* Played, my first Atelier Iris game. Was fun but became a pain trying to craft everything. Overall 8/10. Battle system was ok but kind of boring.
Atelier Iris 2: The Azoth of Destiny. Wasn't super impressed with the first although it was entertaining, have alot of other stuff to play before even thinking of buying it.
Atelier Iris 3: Grand Phantasm.* See above.
Baldur’s Gate: Dark Alliance. Played original on computer, not really into these types of RPG's on consoles.
Baldur’s Gate: Dark Alliance II. See above.
Bard’s Tale, The: Song of the Bard. Haven't Played
Breath of Fire: Dragon Quarter. Most disappointing of the Breath of Fire's I've played. Was basically just decent. 6/10.
Champions: Return to Arms. Haven't Played
Dark Angel: Vampire Apocalypse. Haven't Played
Dark Cloud. Haven't Played
Dark Cloud 2. Haven't Played
Dawn of Mana. Haven't Played
Digimon World 4. Haven't Played
Digimon World: Data Squad. Haven't Played
Disgaea: Hour of Darkness. Best Strategy RPG for PS2. 10/10
Disgaea 2: Cursed Memories. Haven't gotten around to playing, Disagea 1 consumed a ton of my time and I have a bunch of others to play. Maybe down the road.
Dragon Quest VIII: Journey of the Cursed King. Despite people knocking it I love these old school type of RPG's. 9/10.
Dual Hearts.*Haven't Played
Duel Masters. Haven't Played
Ephemeral Fantasia.* Haven't Played
Eternal Ring. Haven't Played
Evergrace. Haven't Played
Fallout: Brotherhood of Steel. Played on Computer, classic. Play it for PC. 10/10
Final Fantasy X. Decent FF but nothing extraordinary. There are at least 4 other FFs I would put before this. 8/10.
Final Fantasy X-2. Refuse to play this fan service, I read the script.
Final Fantasy XII. Currently have it unopened and waiting to play. One of my remaining.
Forever Kingdom. Haven't Played
Front Mission 4. Played Front Mission 3 for PSX and never really got into mech strategy games. Haven't Played
Fullmetal Alchemist and the Broken Angel. Haven't Played
Fullmetal Alchemist 2: Curse of the Crimson Elixir. Haven't Played
Future Tactics: The Uprising.* Haven't Played
Gladius.Haven't Played
Graffiti Kingdom.* Haven't Played
Grandia II. Basically what you would expect from a Grandia game. Nothing great, nothing horrible, just mediocre. 7/10.
Grandia III. Another Mediocre Grandia game. I've become uninterested in this series because the story is usually pretty crappy. Some nice CG scenes though. 7/10
Grandia Xtreme. I think I might have played it when it came out in 01 but I can't really remember it.
Growlanser: Generations.* Haven't Played
Growlanser: Heritage of War.* Haven't Played
Heroes of Might and Magic: Quest for the DragonBone Staff.*Haven't Played
Inuyasha: The Secret of the Cursed Mask.*Haven't Played
Jade Cocoon 2. Haven't Played
Justice League Heroes. Haven't Played
King’s Field: The Ancient City. Haven't Played
Kingdom Hearts. Played, if you can get past Disney character's it's an awesome game with a really indepth Story. 9/10
Kingdom Hearts II. I liked it better than the first. Make sure to watch or read important parts of KH: Chain of Memories first if you don't plan on playing it for PSP. Only Problem with KH games is they are very easy. 9/10
La Pucelle: Tactics. Haven't Played
Legaia 2: Duel Saga. Battle system and story was meh, one of the earlier PS2 rpg's I played. 7/10.
Legend of Alon D’ar, The.* Haven't Played
Legion: The Legend of Excalibur. Haven't Played
Lord of the Rings, The: The Third Age. Haven't Played
Magic Pengel: The Quest for Color.* Haven't Played
Magna Carta: Tears of Blood.* Haven't Played primarily due to the fact I heard loading times are the worst out of almost any game.
Marvel: Ultimate Alliance. Haven't Played
Makai Kingdom: Chronicles of the Sacred Tome. Haven't Played
Mega Man X: Command Mission. Haven't Played
Metal Saga.* Haven't Played
MS Saga: A New Dawn.* Haven't Played
Musashi: Samurai Legend. Haven't Played
Nightmare of Druaga, The: Fushigino Dungeon.*Haven't Played
Odin Sphere.* One on my list, hoping to grab and play it soon.
Okage: Shadow King. Haven't Played
Orphen: Scion of Sorcery. Haven't Played
Phantasy Star Universe. Haven't Played
Phantasy Star Universe: Ambition of the Illuminus.* Haven't Played
Phantom Brave. Haven't Played
Radiata Stories. Played, I liked how they incorporated time of day into events. After a while got tedious trying to complete everything. Battle system wasn't that great imho. 7/10.
River King: A Wonderful Journey.* Haven't Played
Rogue Galaxy. Just finished. Gameplay could have been better but you could basically spam skills to get through 90% of random battles in 5 seconds. Decent story, slow at times but had a couple twists. A ton of sidequest type of stuff and huge amount of time to get 100% completion. 8/10
Romancing SaGa: Ministrel Song. Haven't Played
RPG Maker II. Haven't Played
RPG Maker 3.* Haven't Played
Shadow Hearts.* Never got a chance, started with SH: Covenant.
Shadow Hearts: Covenant. One of my favorite PS2 rpg's, awesome battle system, dark story, I recommend this as one of the top 5 must play ps2 rpg's. 10/10
Shadow Hearts: From the New World. Heard it was alot weaker than Covenant with alot of the same. Haven't picked it up.
Shaman King: Power of Spirit. Haven't Played
Shin Megami Tensei: Devil Summoner. Haven't Played
Shin Megami Tensei: Digital Devil Saga.* Actively Looking for it, one of the last on my list I need to buy for PS2.
Shin Megami Tensei: Digital Devil Saga 2.* Need DDS first.
Shin Megami Tensei: Nocturne. Have, on my list to play.
Shin Megami Tensei: Persona 3. Preordered FES.
Shining Force EXA. Haven't Played
Shining Force Neo. Haven't Played
Shining Tears. Haven't Played
Soul Nomad & The World Eaters. Haven't Played
Star Ocean: Till the End of Time. Another very fun game. Story was horrible, like super cheesy, but gameplay is A+. If your looking for a great battle system and awesome post game this is the one to pick up. With a better story could have been 10/10. 9/10.
Steambot Chronicles.* Haven't Played
Stella Deus: The Gate of Eternity.* Haven't Played
Suikoden III.* Step down from Suikoden 2. Overall good game with good story-telling, too much running around though.
Suikoden IV. Haven't Played, heard bad things about it.
Suikoden V. Haven't Played
Suikoden Tactics. Haven't Played
Summoner. Haven't Played
Summoner 2. Haven't Played
Tales of Legendia. Haven't Played
Tales of the Abyss. Love Tales games, never picked up Legendia though because of some bad things I heard. Love this one and an in-depth new game + system is a pleasant suprise. There are a few bugs but besides that it's a great game. Battle system is one of the best for ps2. 9/10
Tsugunai: Atonement.* Haven't Played
Unlimited Saga.* Haven't Played
Valkyrie Profile 2: Silmeria. Have, on my list to play, love VP1.
Virtua Quest.* Haven't Played
Wild Arms 3. Not much to say other than another wild arms game. If you've played any of the previous and liked them then pick it up. WA games are usually really easy though. 8/10
Wild Arms 4. New battle system that I thought sort of failed imho. Another easy game, they did have a new game + which was good I guess. 7/10
Wild Arms 5. Haven't Played. Was kind of dissapointed with WA4.
Wild Arms: Alter Code: F. I believe this was a remake of PSX. I played WA1 and 2 back on PSX already.
Wizardry: Tale of the Forsaken Land.* Haven't Played
X-Men Legends. RHaven't Played
X-Men Legends II: Rise of the Apocalypse. Haven't Played
Xenosaga Episode I: Der Wille zur Macht. Loved Xenogears and I love story driven games. If you don't mind lots of cutscenes/movies/ and thinking about the story then this game is great. I personally loved it. 9/10
Xenosaga Episode II: Jenseits von Gut und Böse. Weakest of the series. They took out some things from Episode 1 which were a step back. Mainly battle system and no shops. 8/10
Xenosaga Episode III: Also Sprach Zarathustra. Currently Playing, from what people say this is the best of the series.
Ys: The Ark of Napishtim. This was a pleasant suprise but nothing really special. It was short for an RPG and not too difficult, but it's really a fun top-down action rpg if you're into that kind of thing. You could probably rent it and beat it pretty easily and it doesn't really have much replayability. 7/10.
Overall must plays for any RPG fan include SH: Convenant, Tales of the Abyss, Star Ocean, Disgaea 1 and/or 2, Dragon Quest 8, Final Fantasy games, at least one SMT game excluding Devil Summoner (preorder PE3:FES for only $30!) and Kingdom hearts. That gives you a good well rounded gameplay and story type.
After that I would get into some of the other very good games including Xenosaga, VP2 (haven't played yet but I'd imagine it's good), Suikoden 3, Rogue Galaxy, possibly Wild Arms. Other notable games which were fun but not really epic or memorable were Radiata Stories, Atelier Iris, and Y's. I've actually heard the battle system in .hack G.U. is alot better than the old .hack games so if you want to try the series I'd go with those. Personally I can't bring myself to play them after the tedium I went through of the first .hack.
If you're really into strategy RPG's the other Atlus games besides Disgaea are good I hear. I basically went down the path of play the best one they made since I'm not into strategy RPG's that much. But if you really wanted you could play Phantom Brave and La Pucella Tactics also. Then of course there's always Digimon (j/k I should have deleted that form the list -_-).