It's obviously Britney. Her peak wasn't just musical success, it was also to do with celebrity. Adele isn't interested in that so her peak lacks that aspect.
People who downplay Britney's peak make me cringe so much
![Bibliotheque](http://img.photobucket.com/albums/v232/korn0818/ATRL_Smilies_All/atrlers/biblio_zps23dff242.gif)
appreciating facts doesn't make you a 'stan', plus it's not like it matters now because her career is really at a low point.
Their peaks can't really be compared though because the times are so different. Nobody will peak in the same way that Britney did again. The same can probably be said for Adele.