I know everyone has that one RPG they fell in love with that will always have a special place for them.
Maybe it was never popular. Maybe it was a love it or hate it and had mixed reviews. Maybe it was a product of nostalgia and the time of your life you played it.
For me it was Saga Frontier on the PSX.
Box Art
A game where you pick one of multiple characters and play thru the same game world on each one completing a different quest and seeing things from a different angle.
People would say the story is sparse, for me it was enigmatic and mysterious. People would say the battle system was obtuse, too difficult, or required too much grinding. I loved the challenge and getting wiped by it as a younger version of myself who wasn't ready for that level of difficulty, slowly figuring it out just made it all the more epic.
What's your game?
For me it has to be Grandia. This game feels like a pure dose of adventure, and to me it is a nostalgia pill I sometimes take for the trip. That it doesn't get more recognition in the modern day is beyond me.
To a lesser extent, Secret of Mana is also a game I like to redo every other year with different people, and that I know absolutely by heart (can't wait to experience that with my son someday).
Lastly I developed a love/hate relationship with the Trails series back in 2011, and the first game in this series in particular shifted my view on JRPGs forever. It is a bit too recent though to be "that game", but had it be published in the 90's it might have been