Rather surprised there's been no posting of this as of yet: every year John Sickels self publishes his excellent summation of every team's minor league system. Earlier this week he excerpted his Cubs recap.
I highly recommend his book for sure, as each of these write ups is probably a summation of a larger write up.
It is worth a visit, and for those who have never read his material: getting an "A" grade is VERY RARE. He'll B+ some of the best minor leaguers and saves the A's for the superlatives. Needless to say, in the years I've been reading his stuff (since 2008 if you want to know) I've never seen grades like this on any farm system that I can recall, but of course I often read the Cubs and skim the rest.