Everyone at lvl 18 (Don't know how that translates in the real world...over 18 years old?) and unlimited mana. My pick would be 
Kayle with healing and invincibility on her side, but she is still affected by stuns just not damage. My second pick would be 
Karthus assuming people count as champions to trigger his ult, then 1 R to kill a majority of the population. Who do you guys think would be the strongest IRL and why?
nasus can get over 21 billion stacks
In terms of lore unbound xerath and syndra have limitless power at their disposal. Also Karthus' requiem would actually kill everything.
.(and also
)
If you can get 300 stacks at 20 minutes, imagine how many he'd have in a month.
Braum, not well known fact, but his mustache actually protects him from all magic, in order to be fair in the league, Riot had to nerf the crap out of it, but irl he is impervious to magic.
depends on your definition of strongest
able to kill everyone in the world quickly? karthas most strength? darius or braum maybe? best duelist? probably someone with a gun. twitch?
Pre-Shurima Xerath or Syndra.
Unlimited power!
Cho Gath? :0
I certainly think Janna would be an interesting pick IRL.
