The most powerful warrior.

In my opinion, the Archdevil is stronger than the Titan – due to its speed (it can reach almost anywhere) and its immunity to magic (meaning you can't slow it down, blind it, or curse it, etc.). The Titan, on the other hand, can be blocked (although the lightning attacks will still remain, but there's not much you can do about that), and it can also be weakened with spells.
Besides, I think the Archdevil is unfairly overlooked. Yes, it doesn't teleport across the entire map, but Tactics can help with that. Also, it has the highest upper damage among all units. If you increase its lower damage with Divine Power, it will become a real killing machine. However, its major drawback is that it is vulnerable to all magic (including Blindness and Subjugation).