In all actuality, too many of the champion lore texts state that the champion is or is revered as the strongest of them all. If I had to take a guess, though, I would say Swain. Sure, Soraka is a Goddess, sure Jax is undefeated, sure Cho'gath is the ruler of the void, but those don't really change whether or not they are strong. Jax could just be facing low-Elo-scrubs and Soraka could be the goddess that doesn't do anything. Swain friggen smashed his leg to bits as a kid, and just sat there like "WHATEVS BRUH," then took over Noxus. Not to mention, a MASTER TACTITAIN, who says "Oh who am I kidding? I'll just morph and eat you." If something so simple is his plan, and he is a master tactitain, he has to be pretty strong.
I'm sure somebody is going to accuse me of just trying to make my favorite champion seem stronger, so I'll just say it. SWAIN IS NOT MY FAVORITE CHAMPION. I have never even played as him, for that matter.