I've heard nice things about Suikoden, and it being one of the best RPG's on PS1. My friend who is known for never finishing RPG's, played through the whole game without losing interest.
Also, one overlooked RPG would be Legend of Dragoon. To be honest, the character models look really bad, but the pre-rendered backgrounds are beautiful, music is decent, story is pretty nice, and the battles are fun. It's far from perfect, but I enjoyed it quite a lot anyway.
If I had to mention something that wasn't an RPG, or an obvious choise like Crash or Spyro, I'd say Dino Crisis 2. It's more action oriented than Dino Crisis 1, but I think it was, and still is, one helluva game. On graphical level, it looks pretty much as good as Resident Evil 2 or 3. And if you liked Resident Evil, then the first Dino Crisis is worth a try too (it has pretty difficult puzzles though, and thus might block your progress often).
Sharkboy wrote:
Colony Wars is sadly overlooked, probably played that demo more than any other game demo. Really wish it would make a comeback.
Funnily enough, I was just thinking about replaying Colony Wars – Red Sun again after all these years. Right after I finish Phantasy Star IV.