From the new girls, I'd say Ke$ha and Gaga. Disclaimer : I'm not shading either of them nor am I calling them flops/ hasbeens. I'm just saying they're yet to recreate the success of their first few singles, without implying there's anything wrong with that
I feel like Beyoncé peaked with Single Ladies and in general during the I Am...Sasha Fierce era.
And Britney comes to mind since Baby One More Time sold 30 million copies. Gaga is kind of sort of another one but it depends if you consider The Fame Monster her second era or if you clump it with The Fame. She peaked with The Fame Monster.
Beyoncé didn't peak with CIL. By that logic a lot of artists peaked at the beginning, a lot of artists signature/most iconic songs are their first. Britney is the same, yet BOMT was not her peak.
