It seems like the definition of anti-hero is flexible, but if we use the standard definition, a hero with unheroic qualities, I'd say Kenpachi Zaraki from Bleach takes it for me. He was so blood-thirsty and battle-crazed, and yet his fights were always so fun to watch. My personal favorite was his fight against Kaname Tousen, which was the first thing to come to mind at this question. The way the antagonist was given heroic motivations, while the protagonist was given villanious motivations...very few anime fights stand out to me like that one did.
If we're extending the definition to villains or former villains that aid the hero in some way, my personal favorites would be Sesshomaru (Inuyasha), Sir Crocodile (One Piece), or Grimmjow (Bleach again).
NNID: Zephyr25 / PSN: Zephyr--25 / Switch: SW-4450-3680-7334
