Slave gets my vote for best track on the "Britney" album.
But no one track alone could of represented either album, because both albums had a wide variety of genres of songs. The reason Gimme More represents Blackout so well is because most of the songs on Blackout are similar in genre. And I'd say they those MATM as the lead single for ITZ just because they wanted the public to know that "Britney was back", and then once everyone knew they would blow their minds with Toxic.