My vote's for the Steve-O show and the Geico Caveman one. There's a pretty thorough list of the worst of the new fall shows here:

http://www.maximonline.com/articles/index.aspx?a_id=7660&src=dx29

4 of those 10 I heard were supposed to be pretty good. Sam I Am, Back To You, Aliens In America and Private Practice. Looking just at the ads, I'll go with the Cavemen as the worst show. lol