For a Medical aspirant, coaching may be one of the major factors that drives them out of a hometown. So in that regard Tier1 cities like Delhi, Mumbai must be the choice Or you may choose to go to coaching hub like Kota, Vizag etc. Dehradon as I know boosts some of the best schools like Doon School etc which are good for overall personality growth but for a streamed coaching approach although Dehradun must have coaching centres but they would not be as good as Delhi. 

However, Dehradun being a capital town would also have good faculties. In my batch of AIIMS, there are two guys from Dehradun so you can’t say that studying there you can’t reach here.Its individual potential that in the most important.

All the best.