Weakest as in quality or sales/charts/impact?
Because Me Against the Music and Slave 4 U bombed in the US charts, but they did pretty good worldwide I think.
Quality wise would be Womanizer, I love the song, but all her other lead singles >>>
What The Hell, trying to emulate Girlfriend success (thanks RCA), It did moderately well but it was completely the opposite of what the album is and lead the album and era to underperform. And I think it's crap