Definitely a rigid MTB. With braze-ons and TIG welds to mount weapons, like custom 12GA zip gun barrels spread fore and aft, when the zombies try to rush you. Pikes, and machetes too.
Choosing a Walmart bike is suicide. Since most zombies will be former Wal-martians. Some brain activity will trigger when they see you. Slurring "NEXT", "Roadmaster" as they rush you. You become zombie sushi as your fork suspension collapses, and chain snaps.