Depends on whether or not I can use magic. If I'm not allowed to use magic, I pick Draco. While he can use magic, he doesn't often resort to cursing other people. Instead, he usually just relies on cruel insults in order to make people miserable. Even the two thugs that always follow him around don't do much of anything throughout the book series. Dudley, on the other hand, likes to use his fists and brute strength against people weaker than him. Without any magic to defend myself with, I would be completely helpless against Dudley's physical strength. If I can use magic though, I pick Dudley. If he ever tries to pick on me, I can just place a nasty spell on him in order to scare him off. With Dudley easily frightened by magic, I expect I could live in peace without either Dudley bothering me or having to listen to Draco's constant taunts.
Attend a class with Professor Snape where he is being extremely cruel to all of his students or attend a class with Professor Binns on a day when he is being extremely boring.