Kyo said:
Nothing to this day is as anti-climactic as the crazy asspulled, deus ex machina finish of Sword Art Online Season 1, where Kirito magically gains admin powers through the inexplicable help of the previous arc's deceased villain and rags on the absolute bitch of a butterfly fairy lookin ass villain who had just been molesting his girlfriend. Absolutely ridiculous.
I wish I had this vocabulary... shame.
Anyway, as for the OP, any Super fight could fit in the top for "Most anticlimatic fight".
The most climatic one, FOR ME, was Luffy vs Doflamingo due to following it weekly and Doflamingo having, since his first appearence, trigged my interest in an astounding level, so much so I wished to see how the man would react when getting stomped by Luffy and how Luffy and Law would do to defeat him considering he seemed unbeatable by either of them fighting alone.