Honestly, I've always thought The Tarrasque took the cake. I mean, it's a giant invincible dinosaur. I'm sure there are a ton of ways for a DM to employ it effectively in the service of storytelling, but aside from juvenile attempts to inject a game with an OMG AWEXXXOME OVERLOAD!!!, I've really only ever seen it used to sling massive helpings of humble pie. They might as well put it in a chef's hat and an apron.
And then there are all the stupid ways that people come up with to actually kill it. Use Move Earth to make a giant pit, fill it with water, and drown the thing. Create thousands of animated vampiric napkins to take advantage of the grapple rules and kill it with constitution drain. There are treatises out there on how to down it with a party below level 6. To me, the Tarrasque, and everything related to it, just reeks of the number-heavy, rules-lawyering, powerplaying aspects of the game, and I really can't fathom why they ever came up with the thing in the first place.