The most useful unit upgrade.

The most useless upgrade isn't just for zombies, but also upgrading stools to nightstands and peasants to militia (they're already retrained into archers anyway, so let them at least bring in gold).

Upgrading a stool to a nightstand is actually quite useful; wouldn't you be happy if an enemy showed up on week 3 with nightstands and Armageddon?
As for peasants, it used to be true that they needed to be retrained into archers, but since the number of trainable units is now limited, it's pointless. Peasants are the weakest unit in the game, with only 3 HP, but if you turn them into militia, you might get something useful.