It wasn't Wallace Gilbery, it's wasn't BJGE, it wasn't Terrence Newman, and it definitely wasn't Jason Allen or Travelle Wharton. IMO our most under rated off season acquisition last year wasn't even a player, it was our defensive back coach Mark Carrier.
When we talk about coaches who have been able to rejuvenate a player's career naturally Mike Zimmer gets all the credit and rightfully so because he's the one in charge (on the defensive side of the ball) and he's the one with a track record of being able to rejuvenate players under his tutelage in his defensive system. Because of this and Zimmer's reputation I actually think the job Mark Carrier has done this year has been massively overlooked.
In 2011, with virtually the same players, the opposing QB rating for the year was about 5 points higher than it was in 2012 against significantly better QB's (both Mannings, RG3, etc.). It's not only the stats either because if you look across the board (aside from Nate Clements who was trying to adjust to a new position) almost every defensive back on the Bengals roster played significantly better in 2012 than they did in 2011.
Terrence Newman, Adam Jones and Leon Hall (once he got healthy) all looked way better this year than any of them have (not just last year) in multiple years. Chris Crocker actually went from being what Rey Malauga was this year, which is the worst player at his position in the NFL in 2011 to being a better than serviceable player for the Bengals this year.
If you go back and look at how bad our DB's played the first part of the season, especially the first week against Baltimore I think it's remarkable how much they were able to improve throughout the season. I'm not saying Mark Carrier deserves all the credit because I'm sure other coaches contributed as well but he's the primary defensive back coach and he wasn't here last year. IMO the DB group is one of the position groups that improved the most from 2011 to 2012 with mostly the same players. I know people will want to bring up injuries and players getting healthy but I still don't we should discount the job Mark Carrier did this year,and that's why he's my most UNDER RATED off-season pickup this year.