A .38 special/.357 revolver is a must for every arsenal. It will shoot mild target loads and heavy defensive loads equally well. If you don't plan to carry it concealed, buy a 4" or 6" barrel. It will give you a better accuracy at the range.
Personally, if I could only have one gun, it would be a 1911 .45ACP semi-automatic pistol.
In order, here are my guns of choice:
.45 1911
.38/.357 Revolver
12 Gauge pump shotgun
30-06 1903 Springfield bolt action rifle
Ruger MarkII Bull Barrel .22 pistol
Glock 17 9mm
AR15
I have lots of others but those are the essentials for every collection. If you learn to make your own ammo you will be able to shoot about three times as much for about the same price. You don't save any money by reloading. You just shoot more. A regular afternoon at the range is about 500 rounds for me. The cost is about $30 and I won't be shooting many .22's. I can make 500 rounds of a single handgun caliber in a little over 2 hours with a relatively inexpensive progressive press (RL550b).