BTS
TWICE
SNSD
TVXQ
RV
I know you said "a" group or solo, indicating one, but I don't follow directions well
BTS: 2018
TWICE: 2015-2016,2019-2020. TWICE Best girls so they get multiple peaks
RV: 2017. Most of my favorite RV titles are from 2015-16 (ICC, DD, OOTN) but the Perfect Red Velvet and Red Summer was when they peaked in both Red and Velvet
TVXQ: ngl I mainly know the 2012 and onwards work so 2015
SNSD: 2013. Lion Heart was their best korean album but IGAB was okay and L&G was a pop masterpiece. I still play flyers all the time