Since the OP is asking who is the biggest in general (overall), then the only answer should be BTS. If we are talking about just in Korea, I can see why people would say IU since she has been popular since her breakthrough in 2010 (digitals). However, I would say based on who has had the biggest impact in their country, then BTS. They're the only artists that was awarded the order of cultural merit by the president and they have won "lifetime achievement" awards and swept all of the daesangs 5 different times at award shows. They also have been voted by the #1 singer by both entertainment figures and the GP (Gallup) for 3 years in a row which no other singer has this past decade.
This!