My source shows that a lot of people agree on four good HM users: Bibarel, Skarmory, Tropius, and Rhydon. Bibarel is the easiest to obtain, and it can learn Cut, Surf, Strength, Rock Smash, Waterfall, and Rock Climb. Tropius and Skarmory can learn Fly, and Rhydon does anything that the rest might not have move slots for.
TL;DR: Bibarel, a second Bibarel, Skarmory, Rhydon (if you don't have the second Bibarel), and/or Tropius.
Hope I helped!