Wild Arms 1 & 2. Fantasy western theme with Zelda style tool based dungeon puzzles like Lufia 2. Not much more to say about them except they're a ton of fun.
Breath of Fire 3. Good exploration. Also has item based dungeon puzzles.
SaGa Frontier is more gameplay focused and is a pretty decent version of SaGa's stat based leveling and ability system.
Chrono Cross has much better gameplay than Chrono Trigger. It has good puzzles and dungeon design, a better combat system and the element system is pretty fun.
Final Fantasy IX is actually decently difficult compared to the other two psx Final Fantasy games and definitely my favourite out of the three of them. It has a good balance between characters with specific roles but also an equipment based ability system that allows for customization
Final Fantasy V. Even though the playstation port's kind of meh, it's still Final Fantasy V.
Tales of Phantasia but personally I prefer the snes version.