Nick Markakis
Brett Gardner
Troy Tulowitzki
Could go on but these are the guys I thought of at the top of my head.
I like both Markakis and Gardner for this list. I think Gardner is great for the Yankees, as he can play the outfield and hit just about anywhere in the order (save 3-5.) He just seems to do everything right.
Tulowitzki is not necessarily underrated, but I can see why you can make the case. When thinking of short stops, the first name that comes to mind for many people is Hanley Ramirez. While a great hitter, Ramirez should not be playing shortstop, whereas Tulo is a fantastic glove and brings just as much to the plate.
Markakis is great defensively, with a huge arm from right field. He might be the best #2 hitter in the game. He is a disciplined hitter who will hit for average and some power. He has two 20 homer seasons, and now that the Orioles have some batters who are threats behind Markakis, he will have better pitches to hit.
Another player on the Orioles who I think is underrated is Matt Wieters. Yes, his bat has not come alive like many expected it, but Wieters has quietly become one of the best defensive catchers in the game, with a huge arm and he can call a great game. One of the reasons the Orioles took off with Showalter last season was that he allowed Wieters free reign with the pitching staff, and he proved he could handle it.
As for an underrated choice on another club, Bronson Arroyo is a name that comes to mind for me. He's had five straight years of pitching 200 innings. While he doesn't have the greatest numbers, he still goes out and battles the opposing #1 and has performed well as the ace for the Reds.