Who would it be?

Personally, seeing as Malzahar is striving to annihilate Valoran (and probably the entirety of Runeterra) and that he's been forming a massive cult around the Void, I think he has the biggest villain potential of all. What about you?

I'm putting a poll with 5 contestants, here is why I picked those 5:

Swain: The guy has Machiavellian villain written all over him. Who knows where he's taking Noxus?

Malzahar: Already explained.

Mordekaiser: Heavily implied that he's the general of some sort of infernal army.

Veigar: Massively powerful dark wizard who wants to take over Valoran (though more of a comic-relief villain than anything)

Varus: Carries an unspeakably malevolent entity within himself which is probably just waiting for him to be done taking his revenge to take over.

Mordekaiser Or i Report you ...

Malzahar. Swain may boast of the armies of Noxus, but an army of the Void is truly something to behold.

Huehuehuehue

Definitely Malzahar.

Morde, because es #1. HUEHUEHUEHUEHUE

Karthus, or maybe Cho'Gath or Kog Maw.

All three are much scarier than the others, and are at the League for, as of yet, unknowable reasons. While Malzahar may be the harbinger of the Void, that doesn't make him particularly villainous. While I could see Varus as being evil, I doubt he'd have the conviction to destroy Ionia or even Demacia or Frejord for that matter. Viegar is definitely evil, but he's also horribly incompetant (I mean he can't even steal a Zeplin properly, what's he going to do against all of Valoran?)

The only one that leaves is Morde, who (I thought) was a resurected blood knight of some kind. Maybe I have to recheck my lore though.

Hecarim. I think the shadow Isles could prove to be an evil greater than the void.

Veigar obviously