So, some other people here have created a zombie apocalypse arsenal. I said to myself, "Why not hop on the bandwagon?" So here is what I would use in case the dead start rising. I'm currently in the US, and I know a bit about guns.
The AR-15 is an almost universally recognized rifle. It's very common, highly customizable, and easy to use. You can probably find one in every gun store in America. Ammunition is in no short supply, either. If I found a quality one, I'd gladly add it to my arsenal. I'd go for a model with collapsible stock, top rail, and carbine-length barrel and handguard, preferably chambered in .223. A stock Bushmaster Patrolman would be nice.
Mossberg 500 or 590
The Mossberg 500 is a common pump-action shotgun. These too, are modular. 12 gauge shells are common in a variety of loads, from slugs to buckshot. There are also less-common rounds that could work wonders against the undead, such as explosive-filled slugs or Dragon's Breath incendiary (sadly, uncommon). An ideal setup would be a shorter barrel and an 8-round mag tube instead of the stock 5-rounder. A top rail and AR-15 style stock with shell holder as well as a pistol grip would be my dream setup. The US military and various police departments use the 590 or 590A1 variant, an overall enhanced variant with extended magazine tube, pistol grip, and heat shield.
Ah, the Glock. Seen in countless movies, video games, and television shows. They are the sidearm of many American police forces, including the one where I live (Specifically the .45 ACP Glock 21 version). So, where ever you go, you will probably find one. They have an astounding magazine capacity. 17 in 9mm, 13 in .45 ACP, 15 in .40 and .357. There are also many aftermarket accessories like extended magazines or even 100-round drums.