So many RPGs. A few of my favorites.

Final Fantasy VIII - It may be the best looking game on the PS1, and my personal favorite. Unique leveling.

Arc the Lad III - The whole collection is worth it. But the third game had a streamlined polish that is rarely appreciated over the second game of the series.

Parasite Eve - Unique gameplay and weapon upgrading (GUNS). Very adult oriented.

Jade Cocoon - Monster capturing and MERGING. This creates completely new monsters with new stats and appearances. Infinite possibilities.

Honorable mentions: I played these a bit, but never enough to give a full endorsement. But still they are popular picks.

Azure Dreams - Rogue-Lite grid movement monster capture dungeon exploring.

Legend of Dragoon - Timing emphasized attacks. Bad translations.