I would rank the top 5 prospects in this order
1. Arcia
2. Rosario
3. Sano
4. Gibson
5. Buxton
My ranking isn't based on the highest upside or most potential but the most likely to positively effect the major league team based on what they have shown thus far. It is so early on for Buxton and I'm sure by midway through next season he will be number 1 but for now we just haven't seen him yet.
Arcia is major league ready. His bat is there and I don't see how he can't hold done right field or DH. If we trade players like span and morneau, I hope they call him up and let him finish out the year in the majors so he is ready to go by next season