Which music artists peaked commercially in their debut era? List them and keep it cute.

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Britney Spears. That's due to her career being frontloaded though.

Britney
Gaga
Beyonce
Legend X

All peaked commercially with their first era. Gaga also musically.

Lady Gaga

Jennifer Lopez
Britney Spears

Britney Spears. That's what happens when your first album outsells your faves' entire discography.

J. Lo definitely did not. Her peak was around "J.Lo" and her remix album. Not "On the 6".

Gaga... is the clear answer.

Why is this reported, so we can't discuss now the different types of sucess of music artists?

Tragically Liz Phair

From the main girls it's Britney.

Gaga peaked with TFM in 2010
Beyoncé's commercial peak was IASF, and her critical and touring peak happened even later.

Lady Gaga, Britney Spears

Christina was overshadowed by Britney in her first era, while she was the biggest popstar on the planet during Stripped. Also, outside of the US she sold much more with Stripped than with self-titled.
Beyoncé's had several peaks, the biggest one being IASF, so DIL clearly wasn't her peak.

Avril

Britney, Christina, Nicki

Jewel

Iggy

Britney

Gaga peaked in March/April 2010 and surpassed the release of BR in Sept 2010, the then-second highest moment of her career with the Meat Dress. The Fame is much lower, but you all know that because you're chart freaks and are WELL aware that it took Gaga the end of 2008 and majority of 2009 to reach global stardom. Pick and choose when you're wise, aye girls!

I read somewhere Xtina hasn't peaked yet