^^Piece of Me is as iconic as a pile of dog crap on the sidewalk
Britney's truly iconic songs are BOMT, Oops, Slave, and Toxic, in my opinion. Of those, BOMT is her signature.
Gaga has one signature song right now, and that's Poker Face. I think Bad Romance does have potential to be iconic.
Anyway, the Gaga v. Britney argument is ridiculous. Two different artists, two different debut periods.