...um. Lucario, perhaps? It can uh hold things with its paws and is pretty tall.
Size matters not when you lie flat on your back, mouth wiiiiiiiide oooooopen, sweating as a Beedrill inches close to your mouth... and those drill stingers ever so sligthly wave about as the Pokémon flutters, dancing off from your mouth area and
right in front of your eyes...
PMD-wise, the one big issue with dentistry is the potential need for local anæsthetics. While sure, you
can whack a mon on the head with a Hammer Arm so you can operate peacefully, that does risk causing more damage than needed - in particular if you have to hit them on the head. Moreover, having to operate a more complicated mouth case while your patient is unconscious and can not give you feedback like "no, doctor, it's
this tooth, not that tooth!" will sure give you good work prospects for further on the road.
Operating on the mouth cavities themselves, the mons with an advantage will be the ones who can craft and operate tools, or have appendages that can fulfill the tasks, for 1.- pincers (you gotta remove that molar eventually, ideally without breaking anything else close to it) and 2.- brushing (most continuous dentistry work is simply keeping your patient's mouth clean, since they likely won't want to learn to use floss even if it exists in your world).
Bird mons, assuming they are not squicked out with the idea, have a spectacular advantage for dentistry work just as they have in the IRL world, just ask crocodiles and hippos.