Obviously there were way more fights than what I have listed so if you don't see yours then select "Other" and post which one it was. Also, I'd like to hear why you chose a particular fight scene.
Anyway, here are my top 3 (they're all tied for for my favorite):
He-Man vs. the Council of Evil & Skeletor's Regulars - Rather than rely on brute force to overcome his opponents, he turned their advantage of greater numbers against them. I also like that he probably wouldn't have won if it weren't for the other Masters showing up.
Skeletor vs. He-Man & Randor - Probably the best animated sword fight scene of the series. I also liked it because there's a great moment where Skeletor realizes that he now has to fight both Randor & He-man. Instead of immediately running away, he actually attacks them both. What made it even more impressive was he relied primarily on his swords. This was the scene that made me realize what a badass Skeletor could be.
Sorceress vs. King Hiss - This fight showed that Sorceress wasn't just some damsel that had to rely on He-Man for protection. Even though she lost, at least she made a decent showing of her combat skills. I also liked this fight because we got a chance to see other aspects of Hiss' power such as disolving into a bunch of little snakes and reforming in a different location.