Based upon your question, I assume:
1. If I can't have any other firearm, and this is my only defense, then I may need to conceal it at times, so a sidearm will be necessary.
2. If I can't get another firearm, then perhaps repairs and parts could be a problem, so it needs to be very durable.
3. If I can't get another firearm, then ammunition must be inexpensive and readily available, and so the ammo needs to be common.
So, while I'd like to say an AR or a shotgun, seems like the answer is a stainless steel revolver, like a Smith & Wesson or a Ruger. Probably in .38 special / .357 magnum, although most folks would also consider a .44 special / .44 magnum.
I'll say a S&W 686, or a Ruger GP-100, with 4" barrels - they take .38 / .357.