Fitton is the first "bottle" episode, where they just spend the entire time cooped up together in the plane, and it definitely does change their relationships. John Finnemore's notes on each episode are insightful and lovely.
St Petersburg is definitely my favourite for character development. For humour I think it might have to be Ipswich. Or Ottery St Mary.
St. Peter and his soft spot for otters. "You guys. I can't stay mad at you."