Legend of Dragoon is the only RPG I've ever owned and been thrilled to sell halfway through. I played a lot of RPGs in the PS1 days, my standards were low, I was in college and had lots of free time, and I had low expectations for Legend of Dragoon, but I wanted something simple and somewhat fun, and I assumed it would have some decent FMV at least.
What I got was a game that looked like it was drenched in buckets of mud, with brown, green, and grey towns and dungeons that were utterly indecipherable without turning on the arrows to mark doorways. The characters were dull and flat, combat was poorly designed and unbearably slow, and the cool looking dragon transformation pretty much just made your character as functional as a regular RPG character. I read a synopsis of the story later and it seemed like there were some interesting moments developments, but it was the blandest, most poorly designed RPG for the system. Stay far away.
As for the other games, I like them all quite a bit. That said, Legend of Mana has very little in common with most other RPGs, including its own predecessors. The battles restrict you to attacking left or right and healing items are used automatically when dropped, making it feel more like a beat-em-up in combat. You have a couple special abilities and some rare coins that summon, but otherwise there really isn't much use of skills or magic. Crafting is deep, and it's the main way you improve your character, but it really requires a FAQ because it is so obtuse. Also, the story is essentially four unconnected storylines that you progress through and a number of one-off adventures. It's charming and beautiful, but it's much more fun with two players, and it's a game that really makes you work to like it.
Breath of Fire IV is great and has lovely sprites, but it feels unfinished, or at least like it was meant to lead into a sequel.
Xenogears was my favorite RPG for many years, but it has not aged gracefully. I love it dearly, and I love reading about it and talking about it, but it's a bit rough around the edges. It does have a strong, if incredibly convoluted, story that really stands out among PS1 RPGs.
Parasite Eve 1 is a great real world RPG with survival horror elements and a touch of Final Fantasy. Parasite Eve 2 is a good survival horror with RPG elements. Both are good, but 1 has a stronger, more iconic setting in Manahattan, whereas 2's southwestern mining town just isn't very exciting. I also prefer the gameplay in 1 overall, even if it is rather easy.
I agree with everyone who has said that Grandia 1 has aged really well. It's one of my personal favorites, and I love the sense of adventure. It may follow lot's of JRPG tropes, but it rises above them with a charming, likeable cast and excellent gameplay. It also really feels like you are traveling across a large world, since you don't have access to every place you've been at all time.