Eh, I'd have to say Zoids and G Gundam are tied for the position. Zoids was completely unoriginal and uninteresting. G Gundam took all the bad aspects of Dragon Ball Z without any of it redeeming qualities, added big robots, and slapped a 'Gundam' label on it. Complete insult to ye olde school Gundam (what can I say, I'm an olde school kinda gal) and Gundam Wing.
One anime that I enjoyed but was disappointed in was Miyazaki's Nausicaa of the Valley of the Winds. I'd heard so many incredible things about it, and then I saw it, and I thought, "Meh, okay." I liked it, but it was no where near as good as Mononoke-Hime, Spirited Away, or Kiki's Delivery Service. Nausicaa came off as way, way too Mary-Sue-ish. She had no character flaws or development. She started the movie as a perfect being, and ended the movie as a perfect being. Miyazaki has done so much better with his later works.