To keep things pared down, let's stick to RPGs that were officially released in either Europe, the USA or Japan. I know that there are a million Chinese Bootleg games out there that are basically trash on purpose because there is little to no regulation there, but we could go on for years with a pool like that.
For me, I'm gonna say Beyond the Beyond, just to get it out of the way. It was touted as the PlayStation's first RPG, and even comes from the same roots as the Shining series (Shining in the Darkness, Shining Force), but boy howdy what a stinker. The battles are ugly, the text is ugly, the music is forgettable (can't remember a single melody for the life of me), the progression of the plot has all kinds of arbitrary bullshit, and I believe there are a ton of game-breaking bugs too. But to top it all off, the story is like someone trying to explain the plot of an actually good RPG and failing miserably at it.