There was something about the way Rupert Grint delivered the line “You’re gonna suffer… but you’re gonna be happy about it.”

In addition to this, I think it was his facial expression that makes me laugh whenever I come across the scene.
Another that never gets old is 'the amazing bouncing ferret'

I especially a big fan of how disheveled Draco looks when he was reverted to human and him looking terrified. Of course, that famous line he threw at Moody "My father will hear about this" adds to the moment! As does 'Moody' sticking his tongue out at McGonagall's back xD I thought that latter moment was childish but quite hilarious.
For those reasons, I'm pleased both scenes were tied into the movie. Moments such as these are crucial as they add lightheartedness to an otherwise dark movie.