The most powerful warrior.

With the introduction of Arantir, this issue resolves itself. The most powerful unit is the Avatar of Death. If Arantir is level 30 or higher, a very large army is needed to defeat him, and preferably an army of Undead, otherwise, it will constantly regenerate. Of course, there are Cave Lords and others (who else removes at least one unit?) who can eliminate him with a single attack, but this is still an exception.