That is certainly an interesting question. Two guys that have had surprisingly good years come to mind in Wilson Contreras and Wade Davis.
Contreras has been a big reason for the turnaround recently with his .321 average with 7 homers and 19 RBI in the month of July. He is also played well defensively and one of the emotional leaders of the team.
Davis has also been impressive as he hasn't blown a save all year long which has stabilized the back end of the bullpen in 2017.
But seriously though, I agree with cucubbie. Willson Contreras has been MVP so far. Top defensive weapon on the team and an X-factor at the plate. 2nd on team in RBI and 4th in homers in just his 2nd year. Impressive. And it's crazy to think that of all the Cubs' young stars, he was probably the least heralded when he first made it onto the 25-man roster.