From XL: Wallachian Merc Cavalry - Fast and with a good charge and attack. With plenty of Wallachian merc cavalry in the army, Jinettes and Mongol Horse archers are significantly less annoying.

Vanilla VI: Gothic knights. Yeah, the build reqs are a nightmare, and they're slow, but if they are deployed correctly they will break up any defensive line and turn it into pudding. I just love watching them in action.