I think Dwarfs for several reasons.
Slayers, no fear and the power of their opponents in battle. Useful for tying up your opponents bigger units.
Cool war machines, the best in warhammer, flame cannons and cannons can scatter enemies easily.
Good defensive tactics, they’re slow but they can wait for enemies to approach while wearing them down with thunderers and cannons from high up, then take them out with the bigger units. (Effective on Orcs).
Runes, more useful than magic, buy before the battle with points and then make your bigger hero’s more or less invincible.
EDIT: Dammit, not enough room to make it a poll.