Armstrong guns: Their high rate of fire and accuracy can rout infantry units in a very short time
Elite rifle infantry (US Marines, Royal Marines, Infanterie de marine, Imperial or Shogunate Guard): They have a ridiculous rate of fire and accuracy.
Kotesu class ironclads: Really annoying, they make short work of anything that is not an ironclad itself and have huge range.
Explosive shots for ships: Basically, as soon as everyone has this technology, you might as well start naval battles with everything already on fire, it will make little difference.
Torpedoes: 2-3 of them sink anything except for maybe ironclads, but sometimes even them. It takes a lot of micro/luck though to actually hit something.

Some units that have quite the opposite effect.

Gatling guns: Surprisingly mediocre with very little ammo.
Sharpshooters, Tosa Riflemen, and whatever else there is that has longer range than standard line infantry and fancy camo bonuses: Surprisingly useless once the bullets start flying in both directions.