The Agressive trait is useful for conquering, but not overwhelmingly so. It only works on melee and gunpowder units, so mounted units (and later tanks) don't get squat. In fact, most of the Agressive leaders listed here actually have pretty poor UUs, or they dont' come early enough to make a difference. Of cource, there is the exception of the Khan's Kleshik, but they dont' even get the free promotion because they are mounted units.
Of cource, the Quechua is pretty good agains AIs because they don't understand how to counter it, but in multiplayer the Quechua isn't much of a powerhouse. The problem with these guys is that against anything but archers, they're just regular warriors. You can do better with other UUs.
If you want someone with serious steam-roller power early on you're going to want Rome for Praetorians, Mongolia for Kleshik or Egypt for War Chariots. Or, if you want a really early scrap, go for Mali Skirmishers. So, I guess if i have to pick one of those seven leaders presented earlier I'd have to pick Kubali Khan, but he's still hardly my favorite conqueror.