Since this is my choice if you can only buy one, we should start here. A defensive rifle is defined as a rifle that is semi automatic, feeds from a box magazine, preferably 20-30 rounds, firing an intermediate cartridge, and capable of engaging targets up to a minimum of 300 meters accurately.
Semi automatic means the weapon fires one round every time the trigger is squeezed. This allows for rapid fire. Rapid fire is not the best option when compared to aimed accurate fire, but in a close range defensive situation the rapid-fire capability is a necessity. While a lever action or bolt-action rifle can be fired fast in a pinch they are handicapped when compared to a modern defensive firearms rate of fire.
The box magazine does not need to be removable, but it is preferred. Weapons like the SKS often feature fixed magazines, but are still capable weapons. A removable magazine does facilitate faster reloads, but an SKS primed with rounds pre loaded onto what’s called a stripper clip is a capable reloading device with practice.
The ammunition used is considered intermediate. This ammunition is not necessarily high powered, but is capable of taking medium game animals, and is suitable for self-defense. An intermediate cartridge is a step down from a full power round, and features tame recoil. This lack of recoil allows for the weapon to be fired in a rapid and accurate manner. This ammunition varies, but the most common variants are 7.63 x 39, 5.56, and .223. Other rounds that can be considered are 6.5 Grendel, 6.8 Special, 300 Blackout, and 5.45 x 39, although these rounds are not nearly as common or cost efficient.