My top 5 or so are:
Heyward- (already up).I didnt think he could possibly live up to the hype. I was wrong to this point, and will continue to be unless he totally falls on his face in the 2nd half
Carlos Santanna-Playing really well, and nothing whatsoever blocking his way. Posey got MUCH more pub(at least here in florida where he played college ball), coming into the season but he is as good or better imo. Position scarcity also helps move him to the top of my list
M.Stanton-I didnt expect him up till about sept. obviously really killing the ball with 14 hrs. Now I think he has forced them to call him up the minute they can do so and still save money.
Posey- I think he will be really good, but can't possibly live up to the hype imho.(note: thats what I thought about heyward for what its worth) The offensive strasburg. Also he is blocked by molina which hurts
Alvarez-I expected laroche to struggle more than he has and clear the way for him to get a fairly early callup. he hasn't but Alvarez is playing very solid in AAA and has to get the call at some point. Just not so sure it will be for a full time starting gig anymore.
Moustakas-didnt know much if anything about him coming into the season, but he has grabbed my attention(thanks to the cafe).
Strasburg-Tons and tons of hype. The whole world knows about him. If he were cheap I would have drafted him but he was rather expensive, specially for a young pitcher.
Storen-Capps' great start may have hurt his chances for an immediate JUNE 1st callup, but if the nats keep hanging around both him and strasburg may be called up anyways.
C.perez- I know hes already up, and hurt by kerry woods. I see them showing woods to trade him. then the job is all his.
"There was shrinkage!".....(George costanza)