The unit has to be both aesthetically appealing AND effective. For example, the Pitati Archer is a very effective unit, probably more so than the Eagle Warrior (both are very effective at their roles). But it doesn't look nearly as awesome and it doesn't have that cool "I turn my enemies into slaves" mechanic so I consider Eagle Warriors to be "superior" to Pitati. In other words: Style counts (as does effectiveness). Which units make you smile when you use them? My vote is for the Varu because there's nothing quite like overrunning your enemies with a pack of angry, snorting Pachyderms. The Varu has by far the best sound of any unit in the game AND it's incredibly effective (a cheap knight that you can get in the classical era).