Which unit can kill the most zombies per second?

Solution 1:

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.

Solution 2:

This could probably do with an updated answer now that the game has been released along with the campaign mode. The reason soldiers were bad (short range) is still correct, however in the campaign tech tree there are things you can unlock to give your soldiers extra range, so they can out-range spitters with this tech upgrade. If you do unlock this tech option then soldiers are better in the campaign as they are cheaper than snipers and cost less tech points to get to.

None of this applies in survival mode, where snipers are still better.