Originally posted by: EpyonNext
I'm sticking with 6th.
Why? Watch the episode(41) when Ichigo and Byakuya face off on the Bridge. Ukitake says to Byakuya, "Who is that?"
Byakuya replies, "No relation. At least, not to the man who just flashed through your mind, my senior."
Thats what leads me to beleive that Isshin was 6th squad captain. Couple it with the fact that Yourichi calls Byakuya "Little Byakuya", and the evidence that Isshin probably left around the same time as Urahara and Yourichi. Also add in that Isshin was probably much younger at the time and might have looked like Ichigo(remember, they are family and looks do pass on with genetics) you can get a strong case for Isshin being 6th squad captain.
At least thats my theory.