I know I don't know anything about Ardyn but Sephiroth is the One-Winged Angel.

Ardyn can become a ghost, and teleport to deal more damage to his enemies or to avoid attacks.

I don't care if Sephiroth is a 1st Class SOLDIER with infused JENOVA cells that give him some godly powers and weird forms. Ardyn lived for thousands of years due to the curse and still looks humans. He only dies to Noctis.

I can't blame people for bringing Sephiroth up, can I?

Sephiroth is supposed to be the strongest warrior, it all depends on the magic factor, and Ardyn was rejected by the kings... Should... with a big IF, should Ardyn wield the ring... he would definitely win, but otherwise there is no competition against the strongest swordsman