The writers of PiratesProspects.com unanimously rank No. 1 overall 2011 MLB Draft pick Gerrit Cole the best prospect in the Pirates system (assuming he signs, of course). All of them also rank second-round pick Josh Bell between fourth and sixth in the system as well.
It's hard to argue with Cole at No. 1. As a college player, he's further along than 2010 second overall pick Jameson Taillon, even though Taillon, when drafted, was extremely advanced for a high school player. I would probably rank Bell around No. 5 as well - he's miles from the big leagues, but he has tremendous power potential and would be one of the best bats in the system if the Pirates could sign him. My top 10 - again, assuming these guys sign, which obviously assumes quite a lot, in Bell's case - might look like this:
1. Gerrit Cole
2. Jameson Taillon
3. Tony Sanchez
4. Luis Heredia
5. Josh Bell
6. Stetson Allie
7. Starling Marte
8. Rudy Owens
9. Bryan Morris
10. Kyle McPherson