Most of the time we moders here use BMP because that is the type the games support. At BCFiles they use TGA files because that is what the game over there supports. BMP files have the higher bit ratios that the others and there fore store more detail. BUMP files store more but nothing outside of renders can these be used for.
So given a choice I would say go with BMP files, because they contain more detail. We can always covert them if need be, as that is a easy process to do.