Acnologia = Fairy tail
Armagemon/Armageddemon = Digimon
Rules: No island busting
Let's say that Armagemon can also fly
Darn, this is a tough fight... Armagemon was able to take the Giga Crusher and a Supreme Cannon down the throat and be fine, as well as two shotting Omnimon, whom easily killed 1,000,000 Diablomon. Acnologia shrugged off everything that nearly the whole main cast had to throw at it. I'd say Armagemon possibly only due to more screen time enabling us to see more of it's capabilities than Acnologia. But if Acnologia gets any more appearances in the near future, I'd say Acnologia would win.
I agree with you 100%... Acnologia didn't really had much screen time... but I'll say he is stronger than Deliora who could city bust... and Acnologia can island bust with ease... so I hope to see more of Acnologia in the future chapters. but for now I too say Armagemon...