Which unit can kill the most zombies per second?

Snipers are by far the best, simply because of their range-to-cost ratio. They out-range spitters, which is essential. The other units that out range spitters are far more expensive (Titans) or slower (Thanatos).

Soldiers, Archers, and Lucifers can all be easily taken down by one or two spitters unless you micromanage them extensively.