Spoiler: Inconsistency between Bahamut and the 3.56 reveals
By now, it's confirmed that Omega was used to bring down Bahamut. Not only that, but Shinryu was believed to be "more powerful than Bahamut," said by Ilbred. But despite what we've seen from both Omega and Shinryu, I have a hard time believing they're greater than Bahamut in terms of power. I'm not sure how large Omega and Shinryu are compared to him, judging by how big they are compared to Belsar's Wall. Omega is at least bigger than Ultima Weapon and Shinryu seems larger than the first brood (Hreavelgar, Nidhigg, Tiamat) and smaller or equal to Midgardsormr. But neither seem anywhere close to the size of Bahamut, who was MASSIVE. He was much bigger than Limsa itself and flew across Eorzea in a matter of seconds. Bahamut's rampage also changed the shape of the continent. Coerthas's climate changed, The West Shroud was annihilated, massive clusters of corrupt crystals could be found throughout Eorzea, Carteneau is in ruins, and the landmarks from 1.0 were either massively changed or destroyed. The fight scene between Omega and Shinyru was awesome, but I have a hard time believing they're capable of being anywhere near close to being stronger than Bahamut. All they did was make an explosion of light. Bahamut nearly destroyed the whole continent.