Trijicon makes a nifty little fiber optic/tritum open reflex sight for about $500. Don't have to turn it on, no batteries to go dead, small and very lightweight. Should work great for Zombies out to 200 yards or so, as long as we get the slow kind.
ACOG's are excellent also altho frightenigly expensive as well as fixed power. I put a 3x on a M&P 15T and like it alot even though it was about a $900 idea. Can't say anything bad about it once I got used to it. I guess much depends on if you want magnifcation or not.
Si vis pacem para bellum