There's nothing quite so satisfying in battle as seeing the enemy commander's smug expression frozen as his head becomes disconnected from his greasy neck. It's bad for enemy morale, which means it's good for you. Oh, and there's nothing bad at all about pruning the enemy's family tree a bit.
Personally, I like early campagin battles where there aren't many strong walls and in places where there's an abundance of phalanx units (playing vanilla 1.5 mostly). Why is this?
Well, if you let the enemy bust through the stockades, they have no choice but to advance up through your streets. If you've got guys smart enough to point all of their spears in the same direction, you can make some pretty gristly kill zones. Inevitably, the enemy starts to run after they realize that a 1:10 kill ratio ain't gonna work when they only outnumber you 3:1, and the enemy general charges, thus turning himself into a kabob.
I've heard of ballistae, heavy cavalry, elephants, and even falxmen all being used to retire the opposing leader from battle. So everyone, what's your favorite way to kill off the enemy general?