Self explanatory title
My choice is Harry Potter and the Chamber of Secrets, more explicitly, the Game Boy Color version. As a Potter fan, the story follows J.K Rowling's original source material closer than the other versions on consoles and PC (not including the LEGO games for obvious reasons), hell, it was closer than even the movies, and I think being an RPG helped with that because the platformers (PC/PSOne) and Zelda clones (PS2/GameCube/XBOX) took almost too many liberties in the story department, making me want to check out the other two RPG's in the franchise (Sorcerer's/Philosopher's Stone GBC and Prisoner of Azkaban GBA). And as an RPG, it's just a solid turn based game all around.
Though back then, if you knew anyone else into Potter, even by playing the games, they're more likely to mention the games on the console and/or PC, which is why I feel this one is my choice for must underrated RPG.
What are yours?