Anything that Erza is in - Hands down. From her introductory fight against the members of Eisenwald, to beating the crap out of Aria, losing against Jose, her character development with Ikaruga
and Jellal, her WTFpwn of Evergreen, graceful win against Midnight, and drawn out ubermatch against EdoErza . . . There's a reason Erza's almost everyone's favorite character.
Natsu vs Jellal - Mostly because of Jellal's magic, but seeing Dragon Force for the first time was awesome.
Natsu vs Zero - I dunno why no one really liked this fight. Zero was one of the strongest mages so far in the series.
Mystogun vs lazus. Mostly cause Mystogun is the most pwnsome character, stupid other world crap(which was even more pathetic than the skypia arc in OP. Don't get me wrong the op skypia arc was epic, the other world arc was decent too, but the whole idea is kinda pathetic if you catch my drift) ruined him T_T he was so pwnsome. I'm hoping he doesn't come back, so his Epicness isnt ruined.