The most useful unit upgrade.

The most useless upgrade isn't just for zombies, but also upgrading stools to nightstands and peasants to militia (I'll retrain him into a ranged unit anyway, so let him at least bring in some gold).

Added after 6 minutes
Oh yes, and the 6th and 2nd level upgrades for Dwarves are also useless, while the upgrades that radically enhance a unit are useful; for example, upgrades have the greatest effect on Inferno and the Academy; for the rest, 50% can wait for the upgrade.