Archers, useful throughout the early game (where they dominate against order and fair very well against most of Chaos early game) and even useful as cheap support mid-game and late-game, are still useful as cheap counters to low-health units. Through-out the whole game, they make one of the best economy harrasers for order IMHO. Albowtrosses are only better than them late-game, when armour is abundant and if they use walls with shield mode spears to block harrasment.
No other units is possibly more versatile and useable throughout the entire game.
Second place would have to be giants for me. Ultimate damage sponge right there. Kinda makes me feel bad how I use them as human shields all the time. They're possibly the worst unit offense wise, but on the defensive side of the game, they're better than 2 speartons (which is about the same population cost)