Greetings summoners!
The world of Runeterra is a brutal, unforgiving one. It is full of evil monsters that would end all life. But which faction is the most dangerous? Is it the all consuming void? The never ending death of the shadow isles? TEEMO?!? Vote in the poll below!
Overall I'd say the 3 biggest threats are the void,the shadow isles and the watchers
All 3 of them look like they could be extremely powerful
The void doesn't seem that organized,but if malzahars visions are true,then they will come and consume everything.Not to mention that the atmosphere of the void seems to be extremely dangerous itself,what would happen if the voids atmosphere actually leaked into the world of runeterra?that would seriously mess things up
The shadow isles are also quite impressive,I mean bilgewater looks rather helpless against them.The harrowing allows them everytime to raid large parts of the sea and they can't do anything about it. And the more enemies they kill,the stronger they get,the next harrowing will be probably even worse than the previous one. However I'd say the shadow isles are at the moment still not that much of a threat,their growth in strenght is the problem,so they could become extremely dangerous if nothing is done against them.
Lastly the frostguard might be a tiking time bomb. The frostguard themselves might be strong,but I wouldn't consider them much of a threat.Their strenght is probably roughly on the same level as the winters claw or the avarosan(maybe a bit stronger,but not by that much),thats why lissandra doesn't take action. The dangerous thing though would be the watchers.If the frostguard managed to awaken enough of them,then that could cause serious troubles. I mean judging from greyors quotes,they were once able to conquer basically all of valoran with the help of the watchers.
Bonus points go to 
who could be more dangerous than all of the above together if freed from the sarcophagus that is limiting his powers,but his motives were always kinda vague,even before the retcon,so he might not be an actual threat
Veigar. He is willing to crush all for his ideology. Teemo doesn't have enough ambition to do what Veigar does.
Teemo is the worst enemy there is
I'd say it's a tie between the Void and the Shadow Isles. Both are slowly slithering their way in and corrupting the lands, and without the League, little is being done to stop them.
Karthus decides to sing requiem... gg every living thing
The most Dangerous?
Technically... 
... if he decides to take a vacation for a second they all will be DOOMED.
is also Dangerous... i mean, he is stupid, insane, evil, and has tons of magical power... im sure that he could destroy a whole city if he wanted to. Also, is capable of corrupting Bard in theory.
The Frostguard themselves aren't as much a threat as the Watchers. It'd be the Others/White Walkers all over again. Thing is with the Shadow Isles, undeath is another state of existence. A ghastly state, but an existence. The Void wants to wipe you from existence.
where dafuq is shurima? azir is an ancient king with the power to control sand goddamnit. sand is in the soil too. so he could basically use it anywhere
also +
The Shadow Isles mostly mind their own business until the Harrowing. The Void seems to be far more active at actually destroying the world, so they get my vote.
I' m surprised you left out 
Brand. Have you read his lore? He is literally the vessel of a fire demon who's set out to start a burning apocalypse on the world. The League is the only thing containing him (he's not really a faction but he's still pretty dangerous).
While the The Shadow Isles are creepy, they don't seem...evil. They just follow a difference lifestyle which is the afterlife. They aren't so much killing as changing how they live.
They even gave us the Pentakill band so they are not all bad.
So I would have to go with the Void since they just want to kill. Not change the world or change how one lives, but just to eat/destroy it and move on to the next world.
"Muahahahaha... all is proceeding exactly as planned..."
