My favorite PS2 RPG at the moment is Xenosaga.

Another one I like is Grandia Xtreme. It doesn't have the best of storylines, and is a bit of a dungeon crawler, but it's got some great gameplay and insane optional quests including a 100 floor bonus dungeon and the extremely badass Level 200 Zenothlee to fight.

I don't really care for Dark Cloud or Summoner. Suikoden 3 had so much potential, but is ruined by a terrible battle system. It's also pretty unbalanced at the beginning.

Multiplayer games: my favorite is without question Gauntlet: Dark Legacy. That is one hell of a party game. With 9 worlds, 8 starting classes, over 10 secret classes and 99 levels this is my favorite party game. The music is also incredible. You'll just have to go to options and turn down the SFX. They've even got a remixed version of the classic Gauntlet title theme during the fight at the Desecrated Temple. I must say that that is incredibly kickass.

I also recommend Champions of Norrath as another multiplayer game. It's got free online play, so if you want to play multi but have no friends around you can go online with it and play with 3 others. It's hack and slash, but really fun. It's a lot like PSO in that it has 3 difficulty levels: Normal, Courageous, and Champion. There are 5 classes to choose from (Barb Warrior, Wood Elf Ranger, High Elf Cleric, Erudite Wizard, Delf Shadowknight), and levels go up to 50. There are 5 Acts in the game with worlds including an elven forest, a gnome cavern, a vampire's mansion, a tropical island, a desert temple, arctic trenches, undersea caverns, and even the Dimension of Hate itself. My only two complaints with this game are the amount of glitches and the fact you cannot take multiplayer offline characters into online games.