For me right now it would have to be Randal Grichuk. Currently hitting .300 and is 3rd on the team in homeruns and 2nd in RBI's. He is also playing great in CF and has to be leading the league with 5 robbed homeruns.
Another would be Miguel Socolovich. He was brought up to replace a struggling Seth Maness and has a below 1.00 ERA in 20 innings of work.