TW200. It would be simple to maintain, it can go anywhere that dirtbikes or quads can go, great fuel mileage, quiet for sneaking around and so many custom parts out there that it can be outfitted however you need it.
Zombies cannot drive or ride so acceleration and top speed are just not that important.
Finally, it is light enough to go up easy on the bumper hitch on that Stryker.
Funny how the best Zombie Apocalypse bike is also the best RV/camping bike.