2016: BTS/EXO, TWICE
2017: BTS, TWICE
2018: BTS, TWICE
2019: BTS, TWICE / BLACKPINK
2020: BTS, BLACKPINK
2015 - BIGBANG dominated the digital charts that year in Korea with the MADE era. The only thing EXO had them beat that year was physicals but when it came to domestic popularity (Gallup) + digitals, it was definitely BIGBANG.
2016 - EXO was still the best-selling boy group at that time but BTS had bigger hits such as Fire + BST. Although EXO had the most overall physical sales, WINGS was the best-selling album on Gaon with 750K+ sales (1st album to sell 600K+ on Gaon). BST was released in October so it didn't chart high on the year-end digital charts but it has surpassed 2.5M downloads and 100M streams on Gaon. BST is a bigger hit than any song EXO has released except for Growl.
2017 - IDK why people think any other boy group should take this year. SD & DNA were the #1 and #4 biggest idol group hits on the Melon decade chart which proves how massive those two songs are. Plus, BTS were ranked the most popular male idol group on 2017's Gallup chart.