This category is likely close to open. I think the only two players who are entirely eliminated from consideration are likely the two Arizona Diamondbacks who appeared in this year's All-Star Game, A.J. Pollock and Paul Goldschmidt. This does pretty much rule you out as "unsung", even if there's a case to be made that both men deserved even more attention. That's particular true for Pollock, who ranked 6th-equal by bWAR and 8th-equal by fWAR among position players in the entire major leagues. ahead by both metrics of the likes of Andrew McCutchen, Buster Posey and Chris Davis. But as far as the SnakePit was concerned, we were singing his praises loud and strong.
Beyond those two... You could also make a case that closer Brad Ziegler's thirty saves got him sufficient attention, and Patrick Corbin's return from Tommy John surgery also received a good deal of coverage. It really depends on your definition of "unsung hero", and I'm prepared to take much the same approach as the BBWAA does with regard to a definition of "MVP", and leave it up to you to decide. Personally, it's the player whose performance most out-stripped his coverage. So, if your performance was good, and your coverage nil, that could be more worthy than someone whose performance was very good, but who received moderate coverage. It's all very subjective.
Rules are the same as for the previous categories, with nominees on the final ballot to be decided by the number of recs in the comments below. If you see a player you think should make the ballot, give them a rec. If you don't see anyone you like, then feel free to put their name in the subject line of a new comment, and argue for them in the body text. This section will close on Thursday morning, with any ties decided by executive edict, and the top five (or so) going into the final vote that day, which will run through Sunday.