Kingdom of Heaven looked good though, despite Jerusalem's location in the Sahara for some reason.
Braveheart has battles that were cinematically pretty groundbreaking at the time, and few have upstaged them yet.
The Arn movies suffer from too low budget (Sweden, remember) and a fair amount of astounding idiocy. They're still worth watching though, if you can do it cheaply -a fair bit of good kit and nice scenery.
The most convincing "Medieval movie" I've ever seen is "In the name of the Rose", excellent movie but low on epic battles.
I'll heartily recommend the classic "Alexander Nevsky" by Sergei Eisenstein too, but not for its historical correctness. It's actually a pretty fun movie despite (or because of) it's obvious propaganda. It's got Nazi-catholic-stormtrooper Teutonic Knights!