A shot gun of some sort. You don't have the basics unless you have at least one shotgun.
Here is the basic must have list:
1. Shotgun, pump action 12ga is the best all around choice.
2. Rifle, SKS seems to fit the bill.
3. Pistol, 9mm or .45acp 1911.
4. Revolver, .357 mag is a good palce to start.
5. .22lr pistol and rifle, Ruger MKII for the pistol and CZ-452 for the rifle.
With those guns, you are in decent shape for most things. If you have a special need, you can get a gun just for that but the above are guns that everyone could use.