Anthony Gose is at the top of the list and he is there almost strictly because of his speed. The closest comp to him is Jose Reyes in the SAL in 2001, but Reyes had much more pop, but that gives you an idea of his speed upside. But on to better prospects...
Wilmer Flores didn't have a great season in the SAL this year but neither did Ivan Rodriguez in 1989 in the SAL. Flores hit 0.264/0.305/0.331 and Pudge hit 0.238/0.278/0.355. Both were 17 and had similar AB's and production. Pudge hit for a little more power, but it gives you an idea why Flores is a highly touted prospect.
Ruben Tejada finds him in the mix with some pretty good company of Eastern League middle infielders. Rollins, Renteria and Reyes all had good numbers with Tejada having the best OBP, Rollins had the best BB/K rate and the most home run power but they look fairly comparable across the board.
1999 Jimmy Rollins 20 0.273/0.336/0.404
2009 Ruben Tejada 19 0.288/0.351/0.371
1995 Edgar Renteria 20 0.289/0.329/0.388
2002 Jose Reyes 19 0.287/0.331/0.425
Jason Heyward is a hard one to find comps for. Not many 19 year olds put up season's like he did in the Eastern league as a 19 year old. Andruw Jones had similar slash lines in 1996 with 0.369/0.432/0.675 and Heyward had 0.352/0.446/0.611. Heyward didn't hit as many homers but had a better walk rate and struck out less. Jones stole more bags but that is to be expected as Heyward has solid running skills but doesn't project to steal more than 15 bags in the bigs. Just looking at 19 year olds in the Southern league, Jorge Cantu, Joel Guzman, Alex Rodriguez and BJ Upton are pretty much the only guys on the list outside of players in the 2009 season. Going up to 20 year olds, Eric Chavez in 1998 had similar slugging skills and walk rate.
Heyward, Mike Stanton, Starlin Castro, Freddie Freeman, Matt Dominguez and Brett Lawrie were all 19 in the Southern this season. This shows to me that a lot more teams are expecting their prospects to produce quicker and the best are challenged more aggresively than they ever have been. It will be intereting to see how many of these young stars develop into big league stars.
Jaff Decker had a great season as a 19 year old in the Midwest League. I didn't have to look very far for comparisons to his season and many are very good players. His best attribute is his bat and his plate discipline. Daric Barton had a very similar season in 2004. Prince Fielder had a somewhat similar season, although he didn't walk or strikeout as much. Sean Burroughs, Jay Bruce, Adam Dunn, Brad Nelson, Adrian Gonzalez and ARod were also in the group of similar players but Barton was the closest.
Daric Barton 0.313/0.445/0.511/0.956
Jaff Decker 0.300/0.439/0.518/0.958