There is only one Robin Hood movie: Errol Flynn in The Adventurers of Robin Hood. Great cast: Flynn, Olivia DeHaviland, Basil Rathbone, and Claude Rains. Properly keeps it moving, breezy and light-hearted even if Merry Olde England looks a bit Southern California to me.
Let me single out the Kevin Costner version as particularly bad - Costner has a limited range, and being a leader of men isn't within that range (notice how Sean Connery projects that missing quality with only a minute of screen time), not to mention PCing the poor story to death, and Alan Rickman playing the sheriff for comedy relief. But then, I've listed some films that are Not Well Regarded below - it's a matter of what age you were when you first saw the film in question.
Some others of interest:
The Vikings: Kirk Douglas and Tony Curtis dual it out as a very (very!) loose rendition of the Ragnar Brothers.
Ivanhoe: Elizabeth Taylor when she was young and thin. George Sanders as the slimy villian. Another hokey movie from the 50's, but I like it.
Alexander Nevsky: Sergei Eisenstein classic. Music by Prokofiev. Yeah, he seems rather a socialistic kinda guy in this rendition (whaddya want with Joe Steel looking over your shoulder in 1938), but the confrontation between the Teutonic Knights and Nevsky at the Ice Slaughter is fantastic.
Seven Samurai: Akira Kurosawa and Toshiro Mifune. I think Ran will appeal to more people with massive formed units in color moving about the field (absolutely stunning).
Braveheart: okay, familiarity breeds contempt, because I can spot so many inaccuracies, but hey, it's Hollywood.
13th Warrior: yeah, the critics hate it, but it's one of my guilty pleasures.
Monty Python and the Holy Grail: An extremely silly movie that actually understands the legends and history more than might be expected. It was also more influential than you might think since just about every medieval fantasy or history that followed it tried to capture it's murky, dark look.
The Black Shield of Falworth: Try to watch this when you are young (you'll laugh more than be inspired as an adult). It has a special place in my heart, but boy is it awful!