To start with, I looked at each level and found the players in the Twins system who became even marginal major leaguers for a very brief period. Michael Ryan and Brad Thomas are probably a stretch even at that low bar. And I used the level that they played at most. Some of these guys got brief promotions.
At High- A Fort Myers
Low A - Fort Wayne
Rookie - Elizabethton
Based on their actual performances in the major leagues, here is a list of the Twins top ten future major league players after the 2007 season:
There are really only two other candidates to be on that list, Matt LeCroy and Travis Miller.
Here are the top ten prospects in the Twins system after the 1997 season accoring to Baseball America in 1998:
1.Luis Rivas, SS
2.David Ortiz, 1B
3.Michael Cuddyer, SS
4.Chad Allen, OF
5.Torii Hunter, OF
6.Dan Serafini, LHP
7.Jacque Jones, OF
8.Corey Koskie, 3B
9.Matthew LeCroy, C
10.Mike Lincoln, RHP
Missing: Rincon, Redman, Mientkiewicz, Pierzynski,
Misses: Allen, Serafini, Lincoln, LeCroy
That is a pretty good record, considering the uncertainties with prospects. But its also important to remember that while we call it Baseball America's list, it is more likely the list of one of their writers with some input from others. John Manuel did this year's list and I don't think he was around in 1998.
The other thing is that if you look at the order of that BBA list, you don't want to make much of the differences between the number one guy and all the rest. Occasionally there is a real "can't miss" guy like Joe Mauer at the top. But usually these are really lists of prospects with little real difference between the number one player and the number ten. So when John Manuel says Nick Blackburn is the best prospect in the Twins' system, take it with a grain of salt. It means Blackburn has about a 50-50 shot of having a major league career.