C'Mon and Crazy Kids have to be the worst singles by default.
C'Mon was an excellent PROMOTIONAL single, but it was too weak for being a second single. And the single version of Crazy Kids is atrocious*, thank to will.i.am. If it was the album version, it'd probably be third on my list instead of last.
*by atrocious I mean moderately good. Nothing that escapes Ke$ha's mouth can be atrocious. 