the most realistic rank for a full 20+ year career would probably be O-6. that is full bird Colonel for the USA and USAF and Captain in the USN. there is at least one dental corps Admiral at all times who is the Chief of the Corps. there maybe others, but RADM Vinci is the only one i can think of off the top of my head.
if one is political enough and takes the right routes through BUMED, it is possible that you could end up as a dentist in charge of one of the big three Navy medical centers. this is all speculation because of the recent merger of the medical and dental sides of the house. i have been briefed that eventually there will be one parent command for Armed Forces medicine, though each branch will retain their own corps and traditions.