Graham Chapman is underrated.
The other Pythons are much funnier, but Chapman is very important. He's sort of the straight man (ironic, for those in the know) and is, for the most part, the real actor of the group. Not like the other actors aren't amazing. They are awesome. But they are funny and Chapman just plays the character outright, without any intention of being funny while others played around him, and I think he was amazing in that.