I have a backwards compatible Ps3, and I have been researching Ps2 rpgs because i missed out on the era and I am a huge rpg fan, so any recommendations? I do like Tri-ace and am wondering in any of their ps2 stuff was good. Also, I have already beaten Persona 4, Persona 3 FES, and have purchased Dark Cloud 2. 
 
Edit: I forgot to mention that I also have beaten the two Kingdom Hearts games. 
Edit 2: Final Fantasy X was my first choice on my next rpg so with all your recommendations, I will buy that next, and I have already got Final Fantasy XII, keep the suggestions coming!

Final Fantasy X

If you haven't Played Final Fantasy 10 that was my favourite 
Also Dragon Quest for the ps2

I have a giant collection of RPG's for my PS2. Take a look at my list. Though it does need to be updated. I should get on that.

My friends really enjoyed the .hack games. I was fond of Final Fantasy X.

The Ar Tonelico series and .hack series are some good choices. Also, Kingdom Hearts 1 & 2.

The first Disgaea was a great strategy RPG that you'd probably like if you enjoyed the Persona games (lots of character building, leveling items/weapons, tons of classes and skill, etc.).  I probably sunk 100+ hours into that game.  Story is so-so, but the gameplay is incredibly addictive.

the best game ever in my opinion - final fantasy x
 
notable mentions: kingdom hearts 1 & 2, dragon quest viii journey of the cursed king, musashi samurai legend (i found it more charming than most i guess, i really love the music). id also have dark cloud 2 on this list if you didn't have it yet.

Rogue Galaxy, Shadow Hearts Covenant , Dragon Quest VIII

Persona 3
FF X/XII
Kingdom Hearts
Dark Cloud Series
Xenosaga Series.
 
Theres just too many to reccomend

I'd echo the Xenosaga series and add Star Ocean 3. Gotta love tri-ace's combat! 

FFX!

 Ys: The Ark of Napishtim & Kingdom Hearts I / II

Kingdom Hearts and .HACK

" I'd echo the Xenosaga series and add Star Ocean 3. Gotta love tri-ace's combat!  "

There are a huge number of really good RPGs for the PS2. I'll recommend a few that haven't been mentioned here.
 
Arc the Lad: Twilight of the Spirits is a really fun lite-strategy RPG. I found the story here really interesting and there's a great cast of characters to go along with it. Fun game if you like slower paced games.
 
Tales of the Abyss is one of the keynote Tales of... RPGs. If you're not familiar with the series, it has a really exciting fast-paced action RPG style, while still maintaining a great cast of characters and a great story. It's really, really long too...a regular playthrough will take something like 45-50 hours.
 
Tales of Legendia is another Tales of... game, but I'm not as familiar with this one. The battle system is different from Abyss, it's almost like a combination RPG + 2D fighting game, if that makes sense. I've heard that it's quite good, but I haven't played too much of it myself.
 
The Suikoden series is also excellent, though for the PS2 the really good ones are 3 and 5. 4 isn't quite as good. The games have a traditional battle system, but they have really expansive character recruitment options and extremely in-depth storylines involving a lot of politics and intrigue. Don't know if that's your thing, but if so I'd *highly* recommend at least trying Suikoden III, as it is a superb game. 
 
Radiata Stories is a terrific and underrated game. It's by Tri-Ace and has a similar battle system to Star Ocean except without the space opera. Instead it's replaced with goofy humor and a Majora's Mask like day/night system where people do stuff depending on the time of day. Pretty much everyone you meet in the game is recruit able into your party.
 
As others have mentioned, the Xenosaga series is good (especially if you like anime), the Dark Cloud series is a fun action RPG, Final Fantasy X is kind of the poster child of PS2 RPGs (even though I don't find it to be one of the best), the Kingdom Hearts series is kind of juvenile but ultimately good, and there are simply a ton of other superb options for the system. You will have no shortage of fun RPGs to find and play. Best of luck!

Shin Megami Tensei III: Nocturne

Shadow Hearts Covenant

Valkyrie Profile 2: Silmeria 
It's astounding.  

DQ 8.  Best PS2 RPG, hands down.