Talent wise it's Zidane of course. I don't agree that he wasn't as good with us as he was in French NT. Yes, he had the syndrom of under performing against weaker oposition, but i remember many matches when his performances were on par with the performances he has given with France.
This is a tough question, and to give a good answer on it, we must have in mind the time period they spent with us, the titles they won with us, or CL finals or semifinals they played, how instrumental they were in title winnings, and for the midfielder and strikers how often did they score goals.
I think that the most serious candidates for the "prize" are : Nedved, Zidane, Montero, Davids, Trezeguet and Deschamps .
1. Zidane spent 5 seasons with us. He played on 191 matches and scored 28 goals ( according to some statistics he scored 31 goals). During his time, we won two Scudetti, but in his last 3 years we once finished in Intertoto Cup, once we lost the scudetto in last round, and we were second once more.
He has won Toyota Cup, European Super Cup, and Super Coppa Italia. We played 2 times in Champions league final and we lost it both times. Once we made it to CL semifinal where Man Utd eliminated us !
He was concidered our best player while he was in Juve, and we can say he was the leader of our team, especially when Del Piero was injured.
2. Nedved - this is his 6th season with us. He played more than 200 matches and he scored 48 goals so far ( and counting).
He won 4 scudetti ( yes 4 , i don't accept the official 2 scudetti), 2 super coppa Italia, and "played" in one CL final ( he didn't play in the final match, but he was the one who led us there).
He gets many points for staying with us in B, and we can say he was instrumental for our team in these 5 seasons.
3. Deschamps - he was with us for 5 seasons, played around 170 matches and scored 4 goals.
He won 3 scudetti , 1 Champion league, 1 coppa Italia, 2 Super Coppa Italia, 1 Toyota Cup, 1 european super Cup. He lost 2 CL finals and one UEFA Cup final .... impressive !
He was ireplacable in our starting 11.
4. Trezeguet - His 7th season with us. He scored 131 goals so far and is the best foreign striker Juventus ever had.
He won 4 scudetti ( officially only 2 ) , 2 super coppa Italia and played in one CL final . He deserves the epithet " Juve legend" for staying with us in serie B !
5. Montero - played 252 matches for Juve in 8 and a half seasons. He scored 6 goals.
He won 4 scudetti, 1 toyota cup, 1 european super cup , 3 super coppa Italia. He played and lost 3 Champions league finals

!
Together with Ferrara he formed one of the best CD pairs in the world, but he had an awful last 1 1/2 year in Juventus.
6. Davids - played 178 matches for Juve and scored 9 goals. He was with us for 7 seasons and won 3 scudetti and 3 Super Coppa Italia. He played 2 CL finals but lost them. Like someone allready said, he was as worthy as 3 players. He was ireplacable at the midfield and had a big impact on those 3 scudetti we won during his time.
Hmmm, it makes it impossibile to chose one of them. They all won so much and brought so much joy to the fans.
I voted Zidane, but he was just as good and important as Nedved was.
But it's sure all 6 of them were great !