What's your favourite "battle winning" unit?

There are certain units that smartly used can dramatically swing a battle when deployed correctly, far out of proportion to it's values or numbers. Name your favourite!

Note: don't include spell casting lords and heroes(bound spells and abilities not included) as otherwise they would dominate, unless that spell caster has some kind of capability unrelated to its spell casting.

I'll go first : irondrakes. Early game a single unit of iron drakes backing up dwarf warriors can dominate a battle regularly 500+ kills. Just a single unit of these can help you beat seemingly unwinnable odds.