Right now, I'd definitely start Legrottaglie over Mellberg.
Mellberg could end up being the better defender, but right now, Mellberg is not accustomed to the team, and averages one brutal play per game... one cost two points v. Fiorentina, the other nearly cost a goal v. Udinese. Juve can ill-afford another blunder in a critical stage of the game, especially against a team like Zenit.
As for left-back... neither are all that experienced... but De Ceglie at least has some international experience with the Olympics and some U-21 caps... as far as I know, Molinaro has none. Molinaro does have more experience with Juve and more Serie A experience. De Ceglie is a better player though, with a higher potential curve. I think I'd flip a coin, but I'd want it to land on De Ceglie.
Aside from that, the rest is pretty obvious.
Buffon
Grygera-Legrottaglie-Chiellini-De Ceglie
Camoranesi-Sissoko-Poulsen-Nedved
Del Piero-Trezeguet