Life of Brian is almost unanimously the Python's personal favorite, so I'm loathe to disagree with them. Chapman was a functioning member of the group again and it shows in his performance and the quality/maturity of the writing.
As for the early meeting with Gilliam, in was in the mid-60s when Cleese was touring NY with the Cambridge Footlights and Gilliam was a young artist with Help! magazine (founded by Harvey Kurtzman after he left Mad), Cleese did a human comic strip (fumetto) for their May 1965 issue (photo attached).