155mm. The recoil is a little brisk, but it is very versatile. You can hunt with it, easily feeding the whole community if you find a herd of deer. It can even take down the rare and aggressive M1 Abrams, an encounter that will normally ruin your hunting expedition. For self defense, the bore looks like it's about 8 miles wide and will seriously deter any potential aggressor. It will draw a great deal of attention at the range, which can be desirable. There are a large variety of specialty rounds for different applications. It is a great round to have in case of zombies, too.
Seriously, though, I would migrate to .40S&W. However, I love collecting new and interesting calibers. Why restrict myself to just one when I can have so many new and interesting weapons?