If I had to cast a vote for a single champion, I'd go with Malzahar for now. Simply because he's trying to usher a much, much larger evil into the world. Morde/Hec don't make sense as they both seem to feed off of sickness/conflict respectively. Both of which require living humans to leech off of. Also, I can see the Shadow Isles being a huge boon to Runeterra against the Void by providing a literally limitless supply of soldiers. If you're an immortal, undead powerhouse, why would you let some hungry demons take that from you without a fight?

Really though, what are all those summoners at the league up to? What are their real goals in everything? They're comprised of the most powerful mages in the world after all. And we've seen from the JoJ that they're not immune to corruption...

LeBlanc. Not in the strait-laced "oh here's this evil villain look they want to destroy the world we must stop it", but in a more sinister and motivated way. Or just for ****s and giggles. idk.
LeBlanc definitely won't be eating the world any time soon, but she will stir **** up, and that will open the way for weakness on Runeterra for other stuff to go down.

I'm surprised that Swain and Veigar got so many votes, and Varus so few.

Swain is the best villain because I feel he has the best mix of power, treachery, and sympathy. For example, Darkseid from the DC comics is essentially Satan, but the Joker or Lex Luthor is a far more interesting character.

They rely on cunning and other tricks rather than pure brute power. Swain is probably less powerful than a few of these guys like Morde, Veigar, Malz, and probably Varus too. It's his reliance on devious sinister plans that make him a better villain.

Where does it say the Morde is from the Void? It was my understanding that he popped up outta no where from Noxus. IMO I think Hec and Morde would team up and pwn the world

Definatly Hecarim. He soloed demancias armies and destroyed everything in his path.

veigar, if hes the villian we never have to worry about anything, he'll slip on a banana peel and ruin his plans while we sit by and laugh

Malzahar. Swain, even if he can manipulate demonic energies with his bird form, is still mortal and still only has control over a mortal army. Malzahar intends to bring the Void to Valoran, which is why Kassadin is hunting him.

Swain & LeBlanc; who knows what The Black Rose will accomplish?