I know there aren't many Star Wars fans in these parts, but the Qui-Gon Jinn and Obi-Wan Kenobi vs. Darth Maul duel (if you made it that far into the movie) was sweet.

Inigo Montoya vs. Rugen in The Princess Bride because it's just too dang funny.

Maximus in Gladiator has several awesome fights, though my favorite is the one in which he tossed the sword into the stands and shouts "Are you not entertained!!" Awesome.

King Kong vs. three T-Rexes in the Peter Jackson remake of King Kong.

And while it doesn't technically count as a fight, the three-way shootout at the end of The Good, The Bad, and The Ugly.

EDIT: The Achilles vs. Hector fight scene in Troy is also one of my absolute favorites...especially if one counts what happens immediately after it.