Ashley Tisdale: She never had a real hit but I guess "He Said She Said" is the closest to it.
Ayumi Hamasaki: "evolution"
Lady Gaga: "Bad Romance" or "Poker Face"

It is her most successful hit to date and has reached no.1 in over 45 countries, a world record

Britney's is BOMT.

No. It's Crazy In Love, Single Ladies and at a push possibly Baby Boy.

Madonna
1) Like a Prayer
2) Vogue
3) Like a Virgin
4) Frozen/Ray of Light
5) Hung Up/Music

Janet
1) That's the Way Love Goes
2) Nasty
3) Miss You Much/Rhythm Nation
4) Together Again/What Have You Done For Me Lately
5) All For You

Britney
1) Baby One More Time
2) Oops I Did it Again
3) Toxic
4) Everytime/Gimme More
5) Crazy/Stronger/Slave For You

I guess Crazy In Love, but Naughty Girl >>>>

Either Bad Romance or Pokerface.

Toxic.

Music is no.2 for Madonna

Gimme More is no.1 for Britney, Slave is 2, and then BOMT with 3 and so on and so forth

Gaga- Bad Romance.

Madonna has a lot of epic and iconic songs.
But my fav is Hung Up,which was a huge hit.

Britney has so many iconic hits.

BOMT, Oops and Toxic have to be top 3 though.

Bad Romance.

Xtina; her whole discography but mostly Dirrty.