If I was to have just one firearm...I would towards a rifle. Since I would only have one firearm to reload for or purchase ammo for caliber shouldn't be an issue. I would go for a SCAR H. I really like my SCAR 16, but if I didn't have to buy more ammo a .308 would be the way to go. Assuming the different barrel sizes become available. I could SBR - for room clearing. Standard barrel for mid range and long barrel for reaching out. It's kind of a cheat since I would still have 3 rifles in one.
It would be the Jericho 941 in .45 ACP.
It has become my favorite handgun to shoot, its tuff as a tank (weighs in like one) and of course fires the venerable .45 ACP.
Good self defense weapon, accurate as all get out, will keep going and going.
Only drawback, its all metal, heavy to use as EDC. But I would make the sacrifice and carry it even though my pants would ride down around my knees.