Hmmm...I see we all agree on Byaku...No wonder...THE MAN IS GREAT!
He has been my favourite captain from the beggining! He has a really strong ban-kai. But it's hard to choose cause there are other great captains with awsome ban-kais too. I aslo kinda like Aizen but I don't think he can be considered as a captain of the 13 devisions anymore...(well, who cares?!)