Shin Megami Tensei: Nocturne.
Because it shows the sort of innovation and depth that very few other games in the genre do. It's a challenging dungeon crawler, a creature-training game similar to Pokemon, and a game world with so few cliches it's difficult to find a single aspect that's not something you've never seen in a game before.
Oh, and it's one of the few games in the genre that emphasizes gameplay over story. (Which is a good thing, since most JRPG stories are throwaway anyway.) That means no having to sit through oodles of boring dialog to get to the next dungeon. And when you DO get to the next dungeon, you actually have fun with it.
If you're willing to drop $60-$70 on a PS2 game, I can't recommend it enough. And it honestly is worth every penny.
"'Casual games' are something the 'Game Industry' invented to explain away the Wii success instead of actually listening or looking at what Nintendo did. There is no 'casual strategy' from Nintendo. 'Accessible strategy', yes, but ‘casual gamers’ is just the 'Game Industry''s polite way of saying what they feel: 'retarded gamers'."
-Sean Malstrom