oh god! That's such a hard decision! I would choose The Sword in the Stone as I used to have both on VHS but only aquired Robin Hood in dvd, so TSITS is the one I haven't seen in years. Also, I have really warm memories attached to it :)Love, love, love both, though!
I think I would also choose Sword in the Stone. I don't remember a lot about it except Arthur and Merlin being turned into different animals by Merlin's magic. And a frisky squirrel who wouldn't leave Squirrel Arthur alone, haha ;)
Sword in the stone