Theres a thread for bad manga, so why not for animes?
The worst I've watched is Psychic Wars. Its just...terrible. The characters are so dull and 2D, the animations bad and the story kinda piddles around so we can watch 'stunning' monster fights.
Nausicaa was a let down for me. Its not a bad film but it wasn't as good as Cat Returns or Spirited Away.
Theres also Bakugon. Jesus, talk about 20 min long commericials. Collect the cards, the balls ( which would make a neat toy on their own), the accessories. The animation is lacklustre and theres a few funny QUALITY moments to.
And Dinosaur Kings, is the same, you just can't buy the cards! Its just pokemon, which bad cgi dinosaurs with magic powers. Just today a lightning bolt charged a Triceratops, causing it to have some supercharged headbut.