Personally, my ranking would be:
1. Edgar Ie
Ie is a 20 years old, Portuguese centerback. He's 6' tall and IMO, he's the most prepared to make the step up.
From totalbarca: The Guinea-Bissau born centre-back is yet another player who showed tremendous progress during the second half of the season. The 20-year-old hardly played in the first half of the season, as he was sidelined due to injury, but whenever he took to the pitch he looked like a walking mistake, being unfocused for most of the time and shaky in possession. The second round has seen him play much more, either as a centre-back or at right-back covering for Patric. He has been stable, if not spectacular, in both positions. Unfortunately, the second round of matches confirmed that he is extremely fragile, as he could not stay injury-free. Next season he’ll be hoping to earn a starting spot, and above all, stay away from injuries.
2. Rodrigo Tarin
The 18 year old Spanish centerback has been earning rave reviews since he arrived at La Masia from Valencia's youth set up a few years ago. Though he's still playing in the Juvenil A team, I think he could make the step up this year, and I'd rate him as one of, if not the most, talented young centerbacks in La Masia.
From totalbarca: Rodri’ is the standout defender in Spain at his age group and currently the centre-back in the Barcelona academy with most possibilities to reach the first team. He joined La Masia in 2011, and immediately established himself as a star in the Cadet A side of 96” generation. This season he will be playing alongside Munir and Kaptoum with the Juvenil A. Fearless in the tackle, great aerial ability, with an excellent ball output, Rodri has shades of Sergio Ramos in him. He is currently with the Spain U19 national team at the Torneo de L’Alcuda along with club teammates and also talented defenders Xavi Quintilla and Joan Campins, while Juanma Garcia (another very highly rated Spanish youth international) has just recovered from injury and joined the pre-season with Juvenil A. With Jose Suarez in goal and Godswill/Campins, Rodri, Juanma, Quintilla/Fran as the back four, it will be one of the strongest Juvenil A defenses in recent times.
http://www.totalbarca.com/2013/youth/rodrigo-tarin-a-defensive-champion/
3. Xavi Quintilla
Rodrigo Tarin's usual partner in central defense, Quintilla has also played at leftback a lot this past season. IMO, the Tarin-Quintilla centerback partnership has potential to be really special.
4. Franck Bagnack
Bagnack, I think, is really talented. But he's also quite error prone. He's a year older than Tarin and Quintilla, but we have to remember that he skipped a level to the Barca B team. If he manages to play consistently, then I think he could be very successful for the team.
From totalbarca: Bagnack didn’t play as much during the second round as he would have liked, as the rise of Edgar Ié as well as the good chemistry between Sergi Gómez and Ilie restricted his playing time. He did manage to make 24 appearances throughout the season though, and his overall play in the second round was better. He didn’t make as many mistakes as in the first round, and appeared to be more composed on the ball. The 19-year-old still has things to learn though, and next season could be crucial for him. He’s one of those players who seem to be in the line for some first team action.
This is just my personal ranking of the most promising central defenders we have in our youth teams (Juvenil A and up), and I think all four of them could be great successes if they're given the chance to impress. However there are a few other promising young defenders, like Adria Vilanova and Pelayo Suarez, who will probably be playing for the Juvenil A team this season.
(Daley), keep popping up. This year the board changed it up a little by giving us Mathieu. He is by no means a bad defender, but seems to be the fallback option. He did not come cheap either, considering his age (30), which raises the question if we could have gotten better value for our money. Liverpool for example look set to land Dejan Lovren (25) for just 5 Million more (25 Million Euros). And if we don't sign anyone else, we would have questionable CB depth, especially if the transfer ban kicks in. One more signing is definitely required in my opinion and there are better targets out there than Vermaelen, the board just has to take their Marquinhos goggles off and look harder. 
