Superpower is its best bet in almost all cases. Since Emboar is usually Banded or Scarfed, the stat drops won't hurt it much. And it hits hard with 120 base power. This is easy to get in BW2 because it is move tutor, but if this is BW, then you need to breed to get it. While this can cwertainly be done, if you have a high level Emboar and don't want to start training another, Hammer Arm can be used instead.
Low Kick can be used with Flame Charge or Bulk Up sets, where switching is inadvisable. Brick Break can be used over Low Kick if you don't have BW2 access.
Low Sweep can also be used on these sets. While the lower power is noticable, the ability to slow down foes on the switch-in can make it worth it at times.
Focus Blast can be used on a Special set, the thing can run a surprising Specs set after all.
For plain old ingame use, Brick Break is your best option, as it has no harmful effects and doesn't rely on weight like Low Kick does.