My favorite fight would have to be this one between Rogue and Gajeel simply for the fact that the Iron Dragonslayer became an equal rival to Natsu after consuming Rogue's shadows. Greatly improving his popularity after Natsu sent him away on a mine cart during the tag team match the previous day.
Gajeel overcame the challenge by consuming an element completely different from his own to gain the power needed to win against this powerful adversary.
Just look at him, he knows this match is over.
After just gaining these powers, Gajeel even shows a mastery over them that Natsu couldn't when he consumed Laxus's lightning.
The powerful ending move, Iron Shadow Dragon Roar.
Another great part to this incredible fight is the courage of Frosch to stand up to Gajeel and protect his friend. Even with tears in his eyes, he stands his ground until Gajeel tells him he won't touch Rogue.