Display MoreMy opinion is TT is not the biggest international hit, but it was the only song to be a geniune overseas hit.
Those other songs nickle and dime in many coutries without being actually known or comparitively successful in any one country. While TT was a geniune hhuge hit in Japan that paved the way for a new era of Kpop there.
I think, based on the anecdotal evidence Ive seen and removing fan power/mass streaming I give the edge to Boombayah. I think BST got slightly left behind by BTS's other huge songs and that stiffled the long term out of fandom impact slightly.
Edit: to OP agree with everything else but I think internationally D4 takes 2018 by quite a bit honestly.
Fake Love has more worldwide certifications than D4 and has more Spotify streams. Fake Love also charted in 25 countries besides Korea. D4 charted in 11.