Xtincta always had more mature songs compared to Knee. Stripped and B2B are better than any Knee album. But Britney's discography is pretty good as well. She is in third place.
I didn't consider Shakira in this, bc her gems are in spanish. I am talking about the anglo girls.
Gaga's discography is absolutely poor. The Fame/TFMonster is her only good album.
OFC, Knee was in her peak back then. And Stripped generated backlash because of Xtina's visuals, but the songs are absolute gold. The fact that the critics were so influenciated by her image that they buried Stripped don't make Britney's songs any better.
ITZ was a great album, as well as Blackout, but Xtina had Stripped, B2B and her debut album was cute. She wins.