Must have guns hmmm...... I'm going to be as practical as I can when considering "must have" guns.
I think first and foremost in everyone's "must have" category would be some kind of .22lr, rifle, pistol whatever your flavor, so much can be done with this little cartridge most could probably get by the rest of their lives with nothing more.
Secondly, I'd say some sort of shotgun, again in whichever form you desire, quite a bit can be done with a good shotty from squirrels and rabbits, to ducks, geese, deer and just about everything that walks or crawls. It's an awesome HD weapon.
Thirdly, and I'm not so sure it's a "must have" but I know if I didn't have one I'd at least find a way to turn this want into a "need", again whatever handgun suits your needs, when situations won't allow you to function with a long arm slung across your back.
And lastly, trying to be as practical as possible, some sort of high-powered rifle. Unless you hunt large game, shoot far distances for one reason or another, or there is a real chance of fighting other human beings, I really don't see a hi-powered rifle as a "must have" per se.
However, as we are all Americans and knowing our history and being prepared for most possibilities, some people will find they "must have" one for those "possibilities".
There are people that live in areas where a hi-powered rifle would be necessary for warding off dangerous or pestilent animals, maybe even rabid ones. The hi-powered rifle is another tool, you may not need it most of the time, but when you do need it, it is definitely a "must have".