*Update*
I changed my mind after watching a scene from Fate/Zero and it is way better than the Death Gun fight in GGO (SAO 2), although that scene was pretty epic. The best anime fight would have to be the Fate/Zero. The episode which the masters, along with their servants, move in to defeat the mad servant, Caster with his master after the two had went on a child slaughtering spree. It contained sniping, swords, lances, reality plain, a big guy flying in a carriage drawn by oxen, awesome moves, and a freaking giant monster thing that was ripping fighter jets out of the air and eating them. It was a life or death situation as the thing slowly made its way to the cities occupants who were too busy watching to be smart and run away. Gilgamesh vs Berserker fighting in the air. Rider engulfing the monster in his reality. Saber and Lancer trying to fight and then Caster trying to kill everyone. It was a really cool fight.
