Coming from an experienced left 4 dead 2 player : The best weapon I've found in a zombie apocalypse would be the laser sighted AK-47 with extended 40 rounds magazine backed up with a Desert Eagle. Deploy 4 men with such equipment in a diamond formation and proceed to rape anything standing in your way. Including witches and tanks. We managed to down one tank in less than 5 secs. It was beautiful. Shotguns suffer from range, rate of fire, ammo capacity and reload time But that may or may not be the case of the awesome looking shotgun posted by one gentlemen above. Also, real life AK-47s have a reputation of being reliable, ammo is cheap and easy to find. Maintenance is easy and don't require rare spare parts.
In a less video gamish setting I would suggest a good rooftop, rifles of any kind, and a shitload of ammo, dry food, and meds. A couple of flare guns wouldn't hurt either in case somebody out there would be looking for survivors in a chopper. Rifles have the advantage of being extremely accurate even at long range, one bullet to the head is enough so ammo is spared.
Lastly, one of the best ways to handle a zombie apocalypse is to turn the damn virus to your advantage. It's a well known hollywoodian fact that people injected with a crafted breed of any zombie virus using "obscure biological genius worthy mojo" turns you into a AWESOME UNSTOPPABLE KILLING MACHINE OF DOOM. Basically you become immune to the normal devastating effects of the virus, get a bunch of random superpowers often including super strength and regenerative abilities, even one or two "Hideous final transformations" that you may activate at will, or more likely at the climax of the movie when those annoying pretentious "heroes" show up, call you an unnatural evil abomination and proceed to blow you face off or whatever it takes to finish you off. Damn heroes. All we ever really wanted was to be badass and take over a few continents to relive the boredom of being alone in a zombie infested ghost town.
Warning :Left 4 dead 2 related rant ahead. If you recognized yourself as the target of this rant, please learn or never play the game again.
When witches are close TURN YOUR DAMN LIGHTS OFF AND LEAVE HER ALONE!!! You're just going to suck up a med pack, or die if you ever managed to get far enough in an expert game to meet your first witch. Or maybe you just logged into the game and started acting like a jackass... shooting your friends, rushing in melee like the fool you are and wandering off in the wrong direction, begging for a smoker to finish you off and relieve us of a death weight. And then you have the balls to call us noobs and leave the game. Do you know how many times I've backtracked 3-4 rooms to save your useless ass because you couldn't keep up with the rest of the group and get your headset on so we could coordinate properly? Do you know that every single of those times my other teammates told me I should have left you to rot? Well they are completely right.