I had to leave one out of the running, and I never see anyone complain about Surf. It's by far the most used and useful HM. It's also a great attacking move, being the Water type foil to Flamethrower, Thunderbolt, and Ice Beam. As for the candidates, I predict Flash will win because of childhood trauma. I personally think Defog is the worst, being a dumber version of Flash and all.