MerchantCo
Merchant
What's the most powerful unique unit in your opinion? 

Impi upgrades to Lancers, so nah.
Actually they upgrade to riflemen (they broke the standard Pikeman->lancer path)
3.A unique ability that allows you to use it differently (built in normal promotions are nice but if thats the whole advantage than its the same as barbs and upgrading)
Actually they upgrade to riflemen (they broke the standard Pikeman->lancer path)
Except they don't.Impi upgrades to Lancers, so nah.
Do you mean, once upgraded to riflemen, they are still able to attack twice, both range and melee?
I've never been high on the Longbowmen, mainly because the game is really dumb with 3-range attacks with no indirect fire. Just look at any of the dozen "why can't I attack this unit?" threads on these forums to see why Longbows are a lot weaker in game than they are on paper.
My criteria for a great UU is:
1.Built between classical and renaissance (never got the hang of ancient era wars, maybe I should play on marathon)
2.Have either great mobility defense or healing (otherwise it will be hard to upgrade)
3.A unique ability that allows you to use it differently (built in normal promotions are nice but if thats the whole advantage than its the same as barbs and upgrading)
4. Not be a lancer (seriously they are terrible)