When Merlin creates a primitive assembly line so that he can take Arthur on the squirrel lesson, everything's working fine. When the duo return, it's giving Ector and Kay, who brilliantly decided to put a stop to it, the worst Humiliation Conga of their lives.
The scene where they're fish is pretty memorable too, if for nothing but "That's What Makes The World Go Round".
The fact that Merlin beat Mim with the exact opposite method. While she went big and strong, he became the smallest most insignificant thing you could think of, and oh did it pay off.
There's also the fact that he's simultaneously one-upping her Loophole Abuse. Mim set the rules as 1) no disappearing, 2) no vegetable or minerals, and 3) No imaginary beasts like "pink dragons and stuff". When she points out that she never said no purple dragons, Merlin becomes a germ. It's not invisible, just tiny, and it's also not a vegetable or mineral, so it shows her up while still abiding by the rules of the contest.
Archimedes, when he sees Wart in fish form and trying desperately to escape from the pike, immediately comes to his rescue and goes through great lengths to save him.