Here is the list of three guns that I believe should be in everyones safe:
This is just a great all around firearm. It is reliable, useful, fun to shoot, cheap to buy and cheap to shoot. You can get all sorts of accessories for them including decent capacity magazines. It is a great rifle to use for teaching people to shoot.
Mossberg 12 gauge pump shotgun:
The Mossberg pump series of shotguns come in many variants. Modest prices. Classic wood to tactical. Lots of different rounds available. Great for Skeet and Trap as well as home defense and hunting everything from squirrels to deer. Tons of after market upgrades for it as well.
Glock makes a series of 9mm's that have many form factors. They all take the same mags including the 33 round factory mags.
These handguns are reliable, high capacity and simple to use, clean and maintain.
--If I could only have three guns it would be these three.