This is my opinion based on stats that i've looked at and compared over time.
1. BTS - Well I don't think i really need to explain this one
disclaimer: these next two imo are extremely close to each other and the ranking imo could change with each and every cb but this is my ranking after the recent cbs.
Reasoning:
TXT and SKZ are close but I gave TXT the edge cause both their songs this year had higher peaks and peak streams on Spotify global 200 than SKZ's as well as being able to enter US Spotify Chart. Both SKZ and TXT have extremely strong bside streams as well.
Also gave TXT #2 cause atm... Loser=Lover is actually gaining more than Thunderous daily despite being released earlier. TXT also had a higher ML peak than SKZ (5.9M vs 5.4M)
As stated earlier, they are extremely close though.
Reasoning:
Have had constant charting on the Spotify global 200 and have the highest streams on global behind BTS, TXT and SKZ for BGs.
5. Enhypen
Reasoning:
Well they could potentially have the most streamed 4th gen group song in 2021 with a bside and charted on global with their 1st and 2nd cb's imprving with each. Will be interesting to see full album streams