From the commentary in Episode 5, it seems that the most challenging stunts were the blocks at 4:26 (using a mini kicker to reach the 4-meter-high top), the tank at 3:57 (during one attempt he fell and became temporarily unconscious), and the ramp at 1:45.
They thought the loop at 5:17 would be quite difficult (they had never done it before) but it proved to be relatively easy.
There were so many impressive stunts, it’s hard for me to pick a favorite. The opposite 360 backflip at 4:51 looked pretty difficult.
I like the film’s story of a child playing with his toys. (The mother that appears in the film is Danny MacAskill’s mother.)