Britney of course.
Gaga comes second then Beyonce.
Britney became a popstar overnight with that single. The song, the video, the album, the outfits, the lyrics.. in short,.. Everything with the name "Baby One More Time" was iconic. The single and the album will be remembered and that song will always be on lists to come. I am yet to see a debut album and single with THAT impact. The album's first week sales weren't strong but it sold a lot throughout 2 years. (Just like what Adele's 21 is doing right now but better)
Gaga's debut single isn't strong. It was a hit in the US. Then Poker Face happened and boom she was the it girl. The Fame had strong sales but it never topped the charts in the US.
Beyonce's debut (CIL) was big but it wasn't that big. It wasn't the song that established her as an artist.
Christina is 4th. Gennie in a Bottle could've been the big song on 1999 but Britney happened. And from there she was overshadowed until now. It's all just about Good timing too bad for Christina her timing wasn't good. But still, she had some impact.
Kesha is 5th. When I first heard tik tok, I know it will smash. And it did. Animal is a strong album. Her stray of top 10 hits (all consecutive, amazes me. )
Katy Perry is 6th. I never knew I kissed a girl when it was released.

I knew her from Hot n Cold and thought it was her biggest hit (of OOTB era). The album didn't sell that much as well.
Rihanna is last. I love her but let's face it.. Her debut is the weakest among these girls. Pon de Replay was a weak first single. The sales also could sum up to that. I thought that she would be a one hit wonder. I guess not. The impact isn't important. It's the longevity and the continued success. I never imagined "That girl" from Pon de Replay would have 11 number 1 hits and 18 top 10 hits by now. She is yet to have her number 1 album tho.