21

25 is impressive, but it is a lot easier when you are following up a 24 week #1 diamond album that swept the Grammys as was the biggest album 2 years in a row.  

Imma go with 21 just based on the fact that its success was so organic in a way that the industry had not seen in a long time. 
 

21 ran so that 25 could walk

Both are mammoth albums, each with its own set of remarkable achievements. 21 launched Adele into the stratosphere and to have that kind of slow-burn success with 3 #1 singles (arguably 3 of the biggest songs of the century) was so difficult to achieve even then and is now practically impossible in the streaming era. By the time 2015 rolled around, the world was HUNGRY for new Adele and with Hello being the perfect follow-up single to such a monumental era, 25's release was the event of the decade in terms of music. We'll never see anything like it again, it was incredible to witness and enjoy.

21 ofc