Guy Gardener wrote:
One, he's been the king of heaven before and it ended in catastrophe and blood.
Because of the way
he became the king of Heaven. He's learned his lesson in that regard, he's himself now, and he has other angels seeking him out for leadership.
As opposed to either
of the Winchesters who have "hunter" in their job title and "killer of *any* threats to humanity" as their #1 job description. Oh, and both have been demonic at this point, too. Sam being a demon blood-fueled psychopath, and Dean being a demonic torturer currently sporting the symbol of Cain.
So, really not seeing how they'd be a better choice using your criteria. At least Castiel has the power, the balls, and the experience to do the job.
Likewise for Crowley having gained a semblance of his humanity as the king of Hell, and proving he's more competent and desirable for the job than other demons such as Abaddon.