If its the classic slow moving zombies, my weapon of choice will be a M16 or one of its variants. Light, accurate, plenty of ammo available.

A .22lr semiautomatic rifle will also suffice. Same reasons as the M16 plus you can carry a lot of ammo. Also easy to suppress so the zombies wont hear you. If you are going for headshots, the .22 will penetrate the skull and kill the zombie. No recoil and plenty of ammo means you can shoot all day.

I like shotguns but dont think its the right choice for zombie hordes. Low capacity. Most shotguns carry 2 to 8 rounds only. The AA12 and Siaga are the only ones I know that can have 20 rounds or so. But the AA12 is not available yet. Shotguns also have a lot of recoil. 25 rounds is my personal limit. My shoulder cant take more than that.

Ak 47 is also a great weapon. Problem is ammo availability in the Philippines. It cost about 50 pesos per round when its on stock. The Ak2000 might be a better choice. Same gun as the AK47 but shoots 5.56mm rounds. Same bullets used by the M16.