John Cleese left Monty Python before the fourth series and it's often said that his absence left both the audience and Graham Chapman (his co-writer) let down and empty. However when you look at the dates involved, it's funny to think that Cleese re-joined Python almost the minute that final series was complete - the series aired in late 1974 and Cleese was back with the rest of them making Holy Grail for release in 1975. The initial planning and storyline had actually begun after the third series of Python - thus with Cleese included - so his departure from the troupe was basically just a brief hiatus allowing Palin, Jones and co to do more of their own thing.
Anyway - once we get down to the film itself, we're on very safe ground. Cheaply filmed - and indeed most of the funding there was came from rock bands such as Pink Floyd who preferred to give their band-derived income to Python rather than pay 70% income tax to receive it themselves - it can be viewed either as a series of set-piece sketches or as a genuine developing story, punctuated with humour. Either way, the 'African swallow' bit, the 'Knights who say Ni' bit, the 'It's only a flesh wound!' bit, the 'You mother was an 'amster and your father smelled of elderberries' bit - and we could go on - are all not only quotable but almost necessary parts of cultural education in late twentieth century English-language civilisation.
My favourite bit? Probably the 'three questions' routine. Simple premise, but they are able to twist it and get more out of it than you'd ever think possible. Spike Milligan himself couldn't have done any better with it.
That's my favourite bit. What's yours?