I'm partial to Stihl...

And I'd want a long enough bar that I don't have to get very personal with the brain suckers... Tearing limbs off zombies probably doesn't take a lot of HP, so a smaller engine (50cc-ish) for fast handling on a 24" bar would be my ticket...

Then again, a double headed ax is probably honestly a more effective weapon than a chainsaw... As far as lumberjack zombie killing goes that is...