If I recall Sasori was quite short tempered as well when he first appeared with Deidara in the hidden sand village. You remember how he was always saying how he hated to be kept waiting and he always lashed out at Deidara, of course his whole personality changed once he was out of that armored puppet shell and showed his true form.
I still gotta go with Deidara, being able to get a ride from his flying clay birds and having his long range attacks make him a very versitile partner.
Don't judge my replies or my persona, constructed from the vibe and perceptions my avatar gives you. The contents of my reply doesn't represent the character in my ava (despite its origins), they represent the human behind the screen.
Re: if u where an Akatsuki...........
Damn, no Itachi. Anyways I'll go with Sasori, the other three would annoy me to death.