The Life of Brian - no idea if it is "uncommon" but I haven't seen it being mentioned anywhere in ages. It has a weird and macabre but enjoyable mix of super dry English humor, whacky sillyness and actual satire, while still managing to tell a coherent story.
Life of Brian isn't uncommonly known by anyone, it's still widely regarded as the greatest comedy movie ever made. That's no small feat after 50+ years, considering it is just as brilliant now as it was then.
If you want more uncommonly known Python, go for Meaning of Life (out of the three main films it's the odd one out).
Why not go trough the complete Monty's enzyklo Pythonia?
Mony Puthon and teh Holy Grain is epic. Saw it at elementary school ages ago (early 80s) and was looking for iit for ages. (Not speaking English yet, Imtotally forgot the title) Still need to watch Jabberwocky.
When done, Black Adder's Christmas Carol is a belter as well. (The serie is nice as well, as id Red Dwarf)
I's add Idiocracy and Don't look up as well, but alas, both are documentaries by now. :(
For car fanatics, the Taxi series is nice as well.