I'd have to say either a carbine version of an AR-15 or an AK variant, unless the vehicle was something like an M1 Abrams tank. Shotguns sound like a really good idea until you really do the in depth research, then the real negatives come out. Namely weight, magazine capacity, reload speed, and recoil all work against the shotgun, not to mention effective range. An AR can be as light as 6 pounds, offers 30 rounds in a detachable box magazine, a fast reload when that runs out, the ability to put a Zed down past 300 yards, soft enough recoil to make follow up shots lightning fast in close quarters with practice. The weight issues carry over to the ammo also, you can carry a LOT of 5.56x45mm for the same weight as a lot less 12ga shells. An AK offers a lot of these advantages also, especially if the AK74 variant is selected with its 5.45x39mm cartridge. I know a lot of people like the various 7.62x51mm (308 WIN) semi-auto rifles, but here, I'll have to respectfully disagree. These rifles almost universally weigh close to 10 pounds or more, and the much heavier recoil slows the shooter down when follow up shots are needed. The ammo weighs more, meaning you can carry less, and the extended effective range only helps if you can positively identify a target and make hits at those distances.
A story born of personal experience comes into play here. A really good friend of mine loves the 308's and shotguns. He's always razzed me to step up to a "real man's gun." Ok, I have a falling plate rack in my back yard, so we did a test. Semi auto 12 ga vs. AR-15. We set up the plates, got a shot timer out and had a blast knocking steel down. Every single run, I was the faster. The more plates we set up, the further and further I pulled away. If a reload was involved, it wasn't even fair. Ultimately, at the end of the shooting, with each of us expending around 300 rounds, I was still fresh, strong, and ready for more. My buddy was ready for a cold drink and a nap due to the cumulative affects of the recoil and the weight of the shotgun.
Anyway, I hope I've helped, stay safe everyone.
Remember you are the best weapon, the human mind. Anything else is just a tool. Find a weapon, something that hits that sweet spot and train, train, train!
My thoughts exactly.
There's no such thing as the "Best Anti-Zombie weapon"... Why? Because each weapon have its own set of pros and cons. It always depends on the situation...