Oh they both deserve more screen time. Muk was hardly used after it got caught, but I loved its fun personality and such. It's a good battler too, where it was seen in parts of the Indigo and Silver Conference, tanking its way through.
Gible was interesting, it met Ash attempting to master Draco Meteor and other stuff. It was fun to watch how it would use Bite in odd ways, such as biting Conway's Dusknoir in the Lily of the Valley Conference.
When it comes down to it, I prefer Muk. It just kind of wins with personality and battling, IMO.