Some great choices here.

If we're going the King Arthur route, then obviously Excalibur is a favourite (you already mentioned Monty Python and the Holy Grail). But I'd also put in a shout for The Lion in Winter, Richard III (both the Olivier and McKellen performances), Henry V (both the Olivier and Branagh performances). I suppose you could add at least three Macbeths too (from Finch, Fassbender, and Washington). Elizabeth (about Elizabeth I, with Cate Blanchett) is pretty good too. And don't forget The Madness of King George (with the superb Nigel Hawthorne).