Chapters by kills-
No Mercy- 550+
Death Toll- 650+
Dead Air- 800+
Blood Harvest- 500+
Crash Course- 250+
The Sacrifice- 300+ (I just noticed it said "The Passing" Sorry about that.)
[EDIT:] This is based off of three playthroughs for each map and I rounded them together.
Dead Air is the best choice for kills because it has more moments that cause more zombies, such as metal detectors and stuff.