Hanley Ramirez, hands down IMO. To me there is not a more underrated player in baseball than this guy. You don't hear much about him because of where he plays, but to me he's one of the top 5-10 hitters in baseball. Just some background info - He came up in the Boston organization and was rated as their top prospect in 2003, 2004, and 2005 by Baseball America. As an 18 year old he hit .341 with a .957 OPS in rookie ball in 2002. In late 2005 he was the key prospect sent to the Marlins in the Josh Beckett trade. The Marlins installed him as their starting shortstop in 2006 and all he did was hit .292/.353/.480 with 46 doubles, 17 homeruns, and 51 steals and was named the 2006 NL Rookie of the Year. Last year he elevated his numbers to MVP type numbers and hit .332/.386/.562 with 48 doubles, 29 homeruns, and 51 steals, narrowly missing a 30/50 season. Again this season he's off to an unbelievable start with the bat, hitting .327/.402/.602 with five doubles and eight homeruns already and nine steals, putting him pace for 46 homeruns and 52 stolen bases. He's a legit 40/40 man. This kid, IMO, is in the Albert Pujols/A-Rod/Miguel Cabrera class of hitters but you rarely see him mentioned. Considering the ballpark he plays in makes that much more impressive what he's doing.
He's not a great defensive short stop so he'll most likely move to third base sometime in the future but it doesn't matter because he's got the bat to play anywhere. With a new stadium about to be built, hopefully the Marlins can this kid around for a long time. With Ramirez, Hermida, and Cameron Maybin, they have some nice talent to build around.