A couple of other candidates that haven't been mentioned yet...
Excalibur - probably my favourite film growing up. It's an unholy mess and the dialogue is atrocious but it has some great set pieces and looks fantastic. Nicol Williamson is brilliant as Merlin. Probably the best take on the Arthurian legend on screen.
Also stars Helen Mirren, which got me thinking about other films she has been in...
The Cook, The Thief, His Wife & Her Lover is another great one.
O Lucky Man! - the follow-up to If...
The Long Good Friday has already been mentioned.