Christina, since she does so many more.

Nobody Wants To Be Lonely
Lady Marmalade
Dirrty
Can't Hold Us Down
Car Wash
Tilt Ya Head Back
Woo Hoo
Just a Fool
Somos Novios
Tell Me
Moves Like Jagger
Feel This Moment
Hoy Tengo Ganas de Ti

Britney's only released 6 so far (7 after Slumber Party). And Say Something is a #1 solo song, sis.

Xtina. Lady Marmalade alone >

kingternium said it all.

Nobody Wants to Be Lonely >

Christina obviously.