I've only played RTW, M2TW, and ETW, so:

RTW: War Dogs. Put a couple of units of War Dogs in your early-game Hastati/Equites spam stack, and it becomes unstoppable. The only drawback is it takes 2 turns to recruit them

M2TW: Welsh Longbowmen/Saethwyr. Their arrows have greater range than catapults IIRC. Put 3 or 4 in your army, and the enemy will never reach your battle line.

ETW: Grenadiers. It doesn't matter what faction they're from. If you micromanage a couple of units of grenadiers just right, you can annihilate an entire AI-run army. Plus, it is very satisfying to watch half a unit of line infantry be blown to bits.