I'd still take a shotgun first.
But if it HAS to be a center fire, a .45 or .38/.357 carbine. Preferably a pump action. The idea here is cheap and always will find ammo for it. 9mm might also work. 95%+ of the time you'd need a gun in an emergency, it's at ranges of less than 100-200ft, so a carbine like this is perfect.
Taurus makes a nice range of pump action carbines.
Lever actions are also nice, but they are significantly more complex and slower to cycle in general than a pump. Semi-autos are nice but not as a last resort option, IMO.
Note - if we're talking end of the world scenario, though, I'd go low tech and get a simple black powder rifle if only because you can make your own black powder in a pinch and melt almost anything down for a slug.(note - unless I get to keep some reloading equipment that is... heh)
