My vote is monk from age of empires 2. These dudes can walk up and stare at your friend chanting for a bit, then suddenly they turn around and attack you. That’s horrifying.
Basic builder units. You commit genocide against an entire civilization but leave one of these fuckers alive and 20 minutes later you’re facing en entire army.
I’d prefer a commander from Supreme Commander. It can raise an entire military by itself, and it’s much hardier than basic builders. It’s deadly up close. Even if you somehow defeat it, your reward is a nuclear explosion.
I remember the matches where you transport drop you commander into the enemy base, so both your and their commander blow up inside their base. The hardest part was getting the game rules past the eyes of your friends while setting up the game.