Well, since this thread is already boiling down to a list of people's favorite PS2 RPGs (though that was to be expected), I figured I'd mention a place that, while annoying, can sometimes be a cheap place to find PS2 games used: gamestop.com . Yes, the evil empire. I've seen games that go for $15-20 at other places for $4 in their used bin, and a couple of those rare games can be in the realm of $30 or so at GS, assuming they have them in stock.
But recommendations... hah, sure, I'll chime in, too. I'd agree on the Persona games (3FES and 4), and would also recommend, if you like those and want something harder (or without the social link aspect), Digital Devil Saga 1 and 2. The Tales series is also good if you're looking for more action (which you mention in the op), and I can easily recommend Tales over Star Ocean 3. FF12 might work, since it's more action-oriented than 10 (I heavily recommend against 10, BTW.), but it still might be hit-or-miss.
-dunno001
-On a quest for the truly perfect game; I don't think it exists...
