Wow. Azure Dreams and SaGa Frontier are two of my top PSX RPGs...
Azure Dreams is really addicting. It has a Pokemon-esque feel to it. The setting with the tower is kind of lame...but at least it does randomly generate. It's hard at first, but once you power up your little "familiar" or whatever, it's fun. I genuinely thought the game kicked ass.
SaGa Frontier...well...the battle system rocks. For starters, they say there are seven different quests, with them totaling near 140 hours. That's complete BS. If you're a lamer and like to do the same thing over 7 times in one playthrough...then you will get 140 hours; most of that time was based on doing the same thing...seven times. Very few unique areas and such, but still...exploration was what that game was all about. Go anywhere. Do anything. The battles rocked. The combination attacks were devestatingly powerful; the enemy could do 'em, too!
I really, really disliked Vagrant Story. The gameplay sucked so bad, in my opinion. I didn't give it much of a try, but from what I played, a bitter taste was left in my mouth.
Then again, this topic should really be "What, in your Opinion, are the Worst of the Mainstream RPGs?" Most of what people are listing here are well-known and are really only "bad" in respect to the best. If you play some of the lesser-known ones, I'm sure we'd get a lot of new answers here.