I realize this is a slightly unconstrained question (what qualifies as best?), so let's constrain it a little.
In Generation V we've reduced the number of HMs to 6. Now given I can only stick 4 moves on a Pokémon, let's first constrain this problem by limiting the moves to HMs 1-4 (Cut, Fly, Surf, Strength). Now, 3 out of those 4 are physical moves, so let's prefer Pokemon with high Attack. Finally, let's preferentially select for Pokémon we encounter earlier (rather than later) so we can take advantage of our mule of longer.
Best HM Pack Mule that:
- Can use Cut, Fly, Surf, Strength
- Has a high Attack
- Occurs fairly early in Pokémon Black and/or White