The most useful unit upgrade.

In my opinion, the two most useful upgrades are beasts into Furies and Hell Hounds into Cerberuses, because they provide retaliation, and in the case of Cerberuses, also a triple strike. I would put Assassins (due to poison) in second place, and Demonesses (chain lightning) in third. The other upgrades are also good, but in my opinion, they are less useful than these.