Merlin introducing Archimedes to Ector when they first meet.
Merlin: My name is Merlin. Er, this is er Archimedes, a highly educated owl. [Archimedes clears throat, stands tall and thrusts out chest] Ector:Educated owl?! Har-har! A-whoo! Say, that's a good one! [Archimedes flies off to perch on the antler of a deer with his back to the room]
When the fat squirrel pursues Merlin.
"Now look here. I am not a boy..I-I mean, I'm not a squirrel, I'm a boy...I'm an old man."
And the fat squirrel's reaction when Merlin changes back into a human. The younger squirrel's reaction to Wart changing back, on the other hand...
"You see? I'm an ugly, horrible, grouchy old man!!!"
He then barks at her, shooing her up the tree.
And when being tickled he starts to say "I'm a stupid old-!"
The entire wizard's duel, but particularly the part where Merlin turns into a walrus in mid air and lands on Mim.
And the fact that, after doing so, is about to ask her if she's all right.
Merlin: Madam, I have not disappeared. I'm very tiny. I'm a germ, a rare disease, I'm called malagolintomontorosis. AND YOU CAUGHT ME, MIM! Mim:WHAT?!
When the wolf, exhausted after chasing Arthur and Merlin, drags himself to the top of the hill, then sees that they doubled back on him. Heck, EVERYTHING about the wolf. He just can't get a break! He could share notes with Wile E. Coyote.
Kay's reaction to Merlin's summoned blizzard. Inside a castle. In summer.
This face.◊ Wart makes it every time Merlin does something unbelievable.
Archimedes nearly got crushed inside his house as it got shrunk by Merlin.