I agree that Mat would probably win for sheer surprise, but I think Perrin could be the funniest if he actually committed to it. Everyone would expect something practical and understated from him, so imagine him showing up in a completely ridiculous costume and acting like nothing is unusual.
Rand choosing something totally ordinary also feels very in character. After all the attention he gets, spending Halloween dressed as a regular farmer or traveler would probably be the closest thing to a relaxing night for him. Egwene, meanwhile, would probably put way too much thought into the details and somehow turn her costume into an entire historical lesson.
My vote is still Mat, though. He'd probably have the best costume and somehow manage to turn the evening into a gambling opportunity.