For Kelly is.... I don't know. I think Since U Been Gone. And for FOB it is.. Light 'Em Up. I mean it is a good song. But there is another good songs in the album (and it had to be a hits too)
Paris - Stars Are Blind (One of her worst songs, yet her best on the charts)
Rihanna - Fire Bomb & What Now (Both are dreadful)
Lady Gaga - Scheiße, You & I, Born This Way (All are really bad compared to her other work)
Kreayshawn - Has none
Lana Del Rey - Video Games (Not one of her best at all)
Britney Spears - Passenger (Her worst song in her entire discography IMO)
Gwen Stefani - Wind It Up (Her worst single)