Well... lets see "lore" of some skins:
Final Boss Veigar: His power is over 9000 AND that is not even his final form. He is the final boss of the game... so, i guess that he is the strongest of the game.
Super Galaxy Rumble: Based on Gurren Lagann... So, he has the potential to do things like the robot of Gurren Lagann. AND he can do the impossible. (the final form of the robot of Gurren Lagann is bigger than the universe)
Infernal Nasus: Stronger than Nasus, Renekton and Azir... he is a ascended demon and can dance like Snoop Dog.
The Eternum: Creatures from other Dimension.
Galactic Azir: Leader of an alien race with high tech.
Creator Viktor and the Battlecast: Robots OP
Mechas: Gigant Robots.
Pulsefire Ezreal: Time Travel.
My vote is for the Final Boss, followed by Infernal Nasus... But Super Galaxy Rumble has the potential to defeat anyone.
Side Note: Demonblade Tryndamere is just a movie "lorewise".