Can we please stop interpreting champion's ability names as to what they really are?
Nunu does not, despite popular belief, reduce the heat around him to -273 degrees Celsius.
And Renekton doesn't knock dominoes over when he uses his ult.
Viegar does not literally summon black holes and what not. He is a dark mage. He uses dark magic.
His abilities are spells just like Ryze's or Syndra's (maybe weaker), only with fancy-er names.

at least within all the mages for me is veigar, he has being trained for all the dark mages in the world and can control the universe to sort point, even xerath that is a full magic being is in problems specially is we see all his body like is AP hit whit the ult of veigar and he is history

Fiddlesticks, cause he killed/drove mad anyone who went into the chamber, and they could not kill him and could only seal him away..
or Cho'Gath since the league is just entertainment for him...

Nocturne for sure, you cannot fight something you can't see or touch! He appears in dreams so no one who has physical damage can take him, I believe it's the same with magic damage.

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.