Muk's best asset is its attack stat (which is almost twice as high as its special attack) so it is best to use moves with types that are classified as "phsyical" (or use the attack stat to calculate damage) in that generation! I just looked up a list of the physical types so let's hm, see... that would be these types!:
Normal, Fighting, Poison, Ground, Flying, Bug, Rock, Ghost, Steel.
Sludge is a good move for Muk until it reaches level 47, in which it can learn Sludge Bomb. I love having Minimize just to mess with the opponent so I would highly recommend it! I find a highly annoying (and fun to use) moveset to be this:
-Sludge Bomb
-Minimize
-Attract
-Body Slam, ( if you can get it from the move tutor, because it has a chance to paralyze! but if not: ) Dig, or Brick Break for coverage.
Muk has incredible HP and Special Defense, so sending him out against a special attacker and annoying the heck out of them (or straight up sludge bombing) should work pretty well!