Singing: BTS....they have the most unique voices that actually sound good together which is a very difficult thing to happen when you put multiple singers together
Rapping: the rapline of BTS have no competition lyrically or sonically, period.
Dancing: Bangtan (obviously)
Stage Presence: BTS, how they manage to turn every venue into a their own concert speaks volumes (ex. iHeart Jingle Ball)
Literally, bigbang is known for having very unique vocals. See the taeyang and daesung. Not only that GD and TOP have different range and style. 
