Zombies are not interchangeable. A few types break structures or disable your defences, and those are the ones that decide a horde night.
Basic types
Walkers, runners and heavily armoured variants. Each answers to a different damage type, so carry more than one weapon.
Structural threats
Some zombies damage blocks directly, and demolisher-type enemies destroy walls outright. Killing them before they reach a wall is the entire defence.
Special behaviour
Spitters attack at range and cops explode when damaged. Give both space, and finish cops at distance.