Who would win?

The corrupted Princess Luna

or the former Aspect of the Earth gone rogue?

Well, Let's see...

Nightmare Moon's goal was to bring about eternal Night. To this end, she captured Princess Celestia and prevented the sun from rising while attempting to locate and destroy the one weapon that could defeat her. She has the power to move celestial objects (Or at least the moon, since Princess Luna can do that while uncorrupted), perform powerful feats of magic, fly, etc. Weaknesses include lack of foresight, underestimating her opponents (and the power of friendship : ) and overall a debilitating case of arrogance.

Deathwing's goal was to destroy all life on Azeroth (which, let's be honest, is quite a bit more ambitious than NM's goal). To this end, he created a weapon of ultimate destruction (Which was ultimately used to destroy him), caused the Cataclysm, turned his flight against the other dragons, enlisted the aid of two of the four elemental lords, attempted to burn down the world tree, and besieged Wyrmrest Temple with the intent to slay all of the dragon aspects. Powers include fire breath, incredible strength, flight, elemntium-reinforced body armor, and an army consisting of crazed cultists, enslaved elementals, and Black / Twilight Drakes. Weaknesses include a powerful weapon of destruction (which he himself created), apparent unwillingness to get involved in fights himself (Why didn't he just go burn down Nordrassil himself?!), incredible egotism and arrogance, and Old God-induced insanity (Potentially explaining his lack of direct action at Nordrassil)

In a nutshell: Deathwing has all of NM's powers (except the ability to move the moon) and then some. He's bigger, stronger, and reinforced with armor. NM is just a snack.

3197532 You're also forgetting the fact Deathwing can control Azeroth's tectonic plates with a snap of the talons.

3189164
Deathwing has the backing of the old gods and could only be killed with the dragon soul...
Nightmare moon lost to a rainbow and the power of friendship...

I think the answer is obvious.

3380109 agreed

3189164 Azeroth in general seems to be more... polarized than Equestria. The weak people are weaker - Budd - and the strong people are stronger - fucking Old Gods and Titans - so Deathwing will win.

3189164 this is interesting that you chose these two cause they are very similar. Many people who do cross overs with the show and the game imply that Luna's transformation into nightmare moon was influenced by the old gods and like death wing she was slowly going insane.
But if the two were to fight that's hard to say seeing we haven't seen the limit to nightmare moons power.