Fun, huh? Maybe it's because I'm thinking too hard or taking it too seriously, but it's honestly quite a stressful proposition with no simple answer...
Obviously I'd want something of a do-all firearm, but do-all firearms are seldom well suited for any specific use, so there's the rub...
Ah, screw it. The more I think of it the harder it becomes to choose, so I'll just wing it!
Taurus Judge Magnum
It's slim enough to carry, is capable of firing 2 different cartridges which are all quite good, and while 5 shots isn't a lot, it can hold 5 pellets of 000 Buckshot per chamber, which comes to a grand total of 25 .36cal/9mm projectiles per cylinder.
Folks may call me a fool, but whatever, I already own a Taurus Judge Magnum, so I know what these .45/.410 revolvers can do, and I've yet to have any trouble with it.
That being said, I'm glad that I don't have to make any such choice in real life, as that would be very much the opposite of fun.