EVERYWHERE!
a place with strong Pokemon
the elite 4
Seven island at sevault canyon.
victory road is a pretty good place
rt.224
anywhere.
the elite 4 or the places with the highest lvl wild Pokemon in them
Nowhere.
A good place would be 7 island.
weenboob
mt.silver is a good place because it has lvl.50 pokemon there
There are various good places to train flying types in Pokemon Platinum. A good thing to do would be to use the vs seeker.