Jade Cocoon 2 for PS2.
Not sure if it's literally the worst or not (if not it's close), but certainly most disappointing experience I've ever had playing an RPG. Its 'prequel' Jade Cocoon on PS1 was an incredible
game that innovated in some big ways not seen since, and is a solid RPG even if it hadn't incorporated those incredibly fun and unique mechanics. Many compare it to Pokemon because you capture monsters and use them to fight for you, but this game not only did so much more than that, it also isn't meant for young children. You play as the main human character in battles, and summon the monsters you later collect in battle. The monsters you capture can then be COMBINED... in every sense of the word.
While it's been compared to Pokemon's evolutions, this game's version of "evolution" leads to your very own unique creation that is formed strategically by the player. Doing so will combine any 2 monster's stats, average-out their level, combine their elemental affinities, mix specific moves they used (sometimes get new ones), and best of all -- THEIR PHYSICAL APPEARANCE is mixed 50/50 from the two you started with. One monster will have their shape/model used in the combined form, and the other will have the skin/texture featured in the combined form, sometimes with additional slight mutations in color and shape. Then each combined monster you make can be combined with more, creating a completely unique 'family-tree' of sorts, granting the player a unique fleet of monsters right down to their physical forms you customized yourself. You can even further-customize their skins in the late game with certain item drops. As your unique monsters level up, their unique forms grow and become more ferocious-looking. This was so mind-blowing to me at the time, and still is to this day. It truly never got near the attention it deserved, and I wholly-recommend it to anybody looking for an incredible Playstation 1-era RPG.
All this is worth mentioning because in Jade Cocoon 2, literally everything
that made the first game an incredible and unique experience was gone. You technically still capture monsters and 'combine' them, but it gets grossly simplified/dumbed-down; nothing about their visuals changes or is mixed in any way; you either get one form or the other you started with. Also the interesting and dark/adult story themes about death and pestilence from the first game was completely gone, only to be replaced by what felt like a bad, soulless American children's cartoon on Nickelodeon with an unlikable nagging pixie always following you around. Everything just feels lazy and pointless by comparison to the first game.
If this game was released as something completely disassociated with Jade Cocoon and didn't ride the coattails of the first game's success, it would've disappointed me less for sure, but still would've been an equally tedious and joyless experience.
Like I said try to find any worse than Hydlide and Super Hydlide
I've never played these two, but I have
played through their successor 'Virtual Hydlide' on Sega Saturn. While certainly not a great
game and thus gets ripped on a lot on the internet, I actually enjoyed it despite its quirks.