The peasant. I'm pretty sure, that pound for pound, the peasant is the most efficient unit in the game.

For a measly 110 florins (?) you get a unit that can take out almost all spear units in the game, kill cavalry once they are entangled, and fill a variety of other purposes.

God forbid you try and fight peasants with mercenary spearmen if the peasants have one chevron of experience. I couldn't even imagine the slaughter.