The most useful unit upgrade.

If you're not playing with the latest expansion, I think the most useful creature upgrades are:
Elder Gremlins (higher initiative and the ability to repair mechanisms)
Titans (archers)
Furies (high speed and initiative; enemies cannot retaliate)
Royal Griffins (strike from the sky and, overall, high damage output)
Bear Lords (claw attack)
Lava Dragons (extremely high survivability)
Bone Archers (archers and high numbers)
High Druids (very powerful lightning attack)
Battle Unicorns (high effectiveness in combat and blinding).