Neltharion was considered to be the most powerful of the aspects, with Alextrasza following, then Nozdormu, then Malygos and Ysera.
Why?
Because Neltharion was charged with the literal physical creation of the landmasses of Azeroth, granted the power to raise and level mountains, to tear up new volcanoes and scour valleys of his own will. If I understand the Cataclysm event as it's meant to happen, he will be using his sheer strength to shatter the very universal boundaries between the elemental plane and the material plane. Mind you, the elemental plane was something created ENTIRELY by the Titans to imprison the Old Gods and their elemental lieutenants. And he blows it up with his body.
It's entirely probable that the Old Gods have infused Deathwing with even greater power. If we wanted to talk extra-fun speculation, perhaps an Old God has entirely possessed him and now works through him. We know extremely little about Deepholme, where Deathwing has evidently been holed up.
For those saying that you don't think he's all that, think about this:
The reason Blizzard gave for the new Blackrock Spire dungeon was that Twilights Hammer cultists were filling up the upper reaches. You know how they said the cultists got there? Because Deathwing personally bored a gigantic tunnel all-the-fuck the way from frickin' Grim Batol. Those in the know refer to that as 'pants-wettingly powerful.'