If I had to pick one gauge and one gun, it would be a 12 gauge Winchester 1897, simply because that's the shotgun I am most familiar and comfortable with.

Fortunately I'm not restricted to one gauge or gun and I've bought 16 gauge & 10 gauge shotguns because I wanted to. In the case of 16 gauges I've bought them because they were interesting and significantly less expensive than a comparable 12 gauge. I don't expect to buy a 20, 28 or .410, but if I come across a really nice 20, 28 or .410 I might change my mind if the price is right.

I have become addicted to English hammered shotguns to the detriment of my wallet.