The ideal candidate in the current playing list in terms of experience position etc is Xhaka, however I'd not give it to him on principle following his outburst v Palace, let alone his ongoing inconsistency in performances much like Luiz previously. Finally there is the matter that he wanted out last summer and may still depart in Jan or next summer, and I don't want him staying based upon getting the Captaincy.
I'd not make any decision until after the Jan transfer window until we know what our starting XI looks like, as neither Xhaka nor Laca may be starting players if all goes well.