Sasori, all the way. Itachi is cool, but most everybody picks him. When he first popped up, i thought he was neat, but then I saw the Sasori fight that lasted almost an entire season!
Besides, Itachi's Jutsus are unique and cool, but all that ninjutsu gets.. well... boring after a while. On the other hand, Sasori's pupets seem much more origional. Dozens of other characters used fire techniques, genjutsu, and other similar fighting styles as Itachi (even though his were well over 10 levels higher than the others). Yet there are only two other characters that used puppets-- and neither of them could have stood up to Sasori!!
Now don't get me wrong, Kankuro is neat and Sasori's grandmother was a badass, but Kankuro got his ass whooped ten seconds into his fight with Sasori and his granmother couldn't have beaten him without Sakura.
My main complaint is that Sasori was the first in the Akatsuki member to die--TWICE!!! I remember when i heard Sasori was being resurrected and flipped out, yet just a few issues after they went back t that subject, he died again!!! WTF!?!?!?
After Sasori i'd say Nagato/Pein, Itachi, zetsu, and then Tobi (when he was funny). The others were cool, but didn't get my attention as much. The only thing that punk Deiadara was good for was blowing things up.