I have a 16 ga Sterlingworth with Skeet and Modified chokes and a stock bent to fit. Its hard to find anything in the uplands to which it isn't suited. Not to say other guns won't work (I have a bunch) but it is near perfect. My second choice is a Beretta 687 Silver Pigeon sporting with the appropriate chokes.

I have honestly never been a 21 fan. Too heavy, and the action is so long as to look clunky. In my mind, it is the ideal target double (which in reality is it's original intent I think).