I'm going to keep this completely unbiased and list songs that the GP is more likely to remember.
Britney Spears:
Baby... One More Time
Oops... I Did It Again
(You Drive Me) Crazy
Lucky
Stronger
Toxic
Everytime
Me Against The Music
I'm A Slave 4 U
I'm Not a Girl, Not Yet a Woman
My Prerogative
Gimme More
Womanizer
Scream & Shout
Beyoncé:
Crazy In Love
03 Bonnie & Clyde
Baby Boy
Naughty Girl
Irreplaceable
Beautiful Liar
Listen
Single Ladies
If I Were a Boy
Halo
At Last
Sweet Dreams
Love On Top
Drunk In Love
Seems like they are tied right now. We'll see what happens in the future.
Also, just because a song was a hit doesn't mean it will be remembered by the GP. Huge songs are forgotten all the time. In my opinion, songs like Piece of Me, Circus, and Sometimes by Britney won't be remembered by the GP, as songs like Check On It, Me Myself & I, and Diva won't be either. Both girls have many great hits that will stand the test of time. There is no reason to be insulting one because you're a fan of the other. Immature.