1. The fake travel documentary
In 1979, when I went to the cinema to see Monty Python's "Life of Brian", the first item to be shown, following the usual perky and utterly annoying adverts, was a travel documentary about various places in Europe...at least, that was how it started out, apparently. The narrator was clearly John Cleese. His initial commentary was mellifluous and stuffed full of verbose and sweeping phraseology...except that over time one notices a slow warping of the commentary, as he morphs, first to waspishness, then to sarcasm, then on to profanity. He slams Austria, where he manages to commentate on the high suicide rate ("they're topping themselves at a rate of knots") and then arrives at Venice, where his frustrated commentary reaches its first profane apex ("Fucking gondolas!"). After that...well, it's worth listening to, many times over.
I have managed to discover the entire movie online. As this site explains, Cleese wrote the commentary of the entire film, and narrated it over stock footage, in a parody of the cheaply-made banal travel documentaries that used to operate as B movies in many British cinemas in the 1950's and 1960's.

2. The Neville Chamberlain speech recreation
In which John Cleese, directed by the late Peter Cook, attempts to re-enact the famous appearance by Neville Chamberlain on his return from visiting Adolf Hitler in 1938. As you will see, he starts off poorly and then gets steadily worse...