Since Omega wants to find the mightiest being, I think it will be:

1- Jenova
2- Sephiroth > Bizarro> Safer Sephiroth (Savage) > Shirtless Sephiroth, so we all perform Omnislash, would be epic.
3- Gaia: Diamond > Ruby > Under Water: Saphire + Emerald > Omega Weapon (Savage)
4- Omega