the strongest unit in the game is the Azure Dragon; if it's a creature versus a creature without heroes... Archangels are NOT the best, because Archdevils go first, and Archangels don't counterattack... But in general, they are all good; you need to look at each creature individually... For example, a Black Dragon will never win against a Behemoth, because it has -80% to defense, but an Archdevil might defeat a Behemoth, and a Titan probably will too... In my opinion, Behemoths and Titans are the best (because they can shoot)...
P.S. And who are these Azure Dragons? I don't remember them...