I will admit that on one level, it is fairly pointless to address a single shotgun as “my favorite gun.” Different shotguns have different purposes, and there is no reason to select only one. Nevertheless…
The first thing you might look for in a single shotgun is flexibility. An 8# sporting clays gun -- and I have several-- is a fine thing, but not ideal for all-around bird hunting. Likewise, a 10 or 12 gauge semi auto -- and I have one-- may be the best choice for waterfowl, but a little style might be satisfying as well. Too much style, of course, is liable to be pretty expensive, and perhaps not the right gun if you care about getting it a little banged up in upland hunting.
In any case, while I have quite a few guns, my favorite all-around is probably my 20gauge Browning BSS Sporter with Briley thinwall chokes and better-than-average wood. With mod/full chokes (or others if you prefer), I can nail longer shots at sporting clays (sometimes), cylinder works for skeet, c/sk for quail or woodcock and so forth. And hevi-shot (or similar) would work for pheasant. It would not work for waterfowl, but that's not what I do.
I don't suggest one gun would work for everything, but this is pretty decent for most things, easy to carry and a SxS is, well, pretty stylish and fun, too. If you give me another gun, I'll take my Browning Superposed Lightning. I have a dedicated modern sporting clays gun, and if I were shooting for money -- not likely-- that's what I would bring, but there is a lot to be said for a neat well-balanced gun with some history behind it.
How about you?

