Cannavaro - fantastic leader and our most consistent performer, but hasn't been in the team long enough and isn't really the future of the team.
Thuram - great presence on the pitch, solid performer, has been with us for some time. Unfortunately he's not getting any younger, so he's in the same boat as Cannavaro. If Del Piero were to (hypothetically of course) leave at the end of this season, I think he should get the captain's armband.
Zambrotta - very consistent performer and makes a huge impact on the team, but IMO is much too quiet to be a captain. I haven't seen him speaking a single word on the football pitch. He's the type of player who just does his job on the pitch.
Buffon - Buffon has been with us for some time and will probably stick around for many more years barring a mega transfer. Another consistent performer and probably the kind of player who inspires his team and makes them feel secure, but personally I don't really like the idea of having a goalkeeper as the captain. Shouldn't count that against him though, as that's just my opinion.
Nedved - next in line for me after Thuram, but I can't see him staying with us for any longer than one or two more seasons. I hate to say it, but his influence has waned significantly compared to his Ballon d'Or winning season. Like Zambrotta though, Nedved is a bit too quiet and business-like to be the captain.
Zlatan - too temperemental. Cantona's an exception. He's the kind of player we look like basing our team around in the future, so If he can clean up his act a bit and start showing some leadership qualities, I wouldn't be against giving him the captain's armband. That'll take at least 3-5 years though I think.
Emerson - another new player to the team, and again he's not getting any younger. A great influence on our team though, and the kind of player teammates feel secure with.
Thuram
Nedved
Cannavaro
Emerson