The most useful unit upgrade.

I initially wanted to include the Mountain Guards in the poll, but then I thought that adding them to the hit points wouldn't be that unique: Imps gain 50% health when they become Devils, and Peasants even double their HP. There are similar examples even among more advanced units (which is much more valuable for the game) - Mages, Hydras, Lizard Riders, Armored Knights, Monks... And I wanted to make the poll as short and readable as possible...