The most useful unit upgrade.

In my opinion, each upgrade is worthy of being the first, but the most important upgrades are Titans, Furies, and Archangels. The other upgrades, under certain conditions, do not provide as much of an effect as these. For example, Demonesses against Inferno, Assassins against Necropolis. Some occupy an intermediate position, where we don't observe any particular changes, but the effect is still there.