It's weird how bent out of shape some people are getting over a completely hypothetical topic. It's rather sad how a few are taking such a condescending tone with others about it (well, it's par for the course with one person, so not surprising there; I'll let others guess who).

I'll add this...

Part of the issue people seem to be dealing with is each one's personal definition of "destroying the world". So, I won't even go there. I'll just speak to the overall amount of destruction each creates, and the fallout from it.

In Deathwing's case, he messed some places up, while entire other regions are left untouched. So his swath of destruction was very much limited to only certain parts of Azeroth. Further, cities are left standing in each case with only some damage done. All in all, the impression I had (even at Cata's launch, well before anything to do with XIV or Bahamut), was that he was a pissed off dragon going on a temper tantrum.

It's accurate to say that in the 2.5 years or so that he was freed, he failed to destroy the world. The worst of his destruction was inflicted when he first emerged.

On the other hand, Bahamut manages to change the face of pretty much all of Eorzea within minutes of emerging, changing entire climates in some cases from what we've seen. And I don't mean "making a long trench here, or a large crater there", I mean rendering the world pretty much unrecognizable to anyone who saw it during 1.0. Outside of the cities (which seemed to have been mostly spared, just like in WoW).

That said, we don't know how much impact his attacks had on the cities themselves. While none may have suffered a direct hit (which remains to be seen), that's not to say the fallout couldn't have been equally devastating - choking smoke and ash and debris, etc...

And then there's the matter of the folks in XIV having to be actually teleported away to safety, or face certain annihilation. We don't know if that was everyone on Eorzea, or just those immediately within and around the cities. That remains to be seen... and perhaps something in ARR will explain all that.

I would have to say, with what we've seen of ARR (screenshots, videos, etc) and what we can see of Azeroth, and considering the effects each attack had on its respective world's population... I'd have to give it to Bahamut.

It seems pointless to talk about what happens to Bahamut after his attack, or his imprisonment in the labyrinth, because we don't know how he ultimately ends up there, yet. There's a lot of guessing and theory-crafting, but no one actually knows at this point (well, outside of SE anyway).