I Think We Can All Agree Now...

(Puts on nerd glasses)

Well you're not wrong. Even a fire elemental follows the rules of physics. The color of his flames tell us how hot he is, becuase different particles start combustion at different temperatures, changing the color of the flames.

His flame is a red-orange, so we are looking to temperatures ranging from 980 degrees (barely visible red from the bare model) to 2,200 degrees F (a clear orange with barely any red.)

The sun however starts at 10,340 degrees F at the surface, which I will assume our fair sun maiden draws the line to avoid cooking her ADC.

But as it turns out we're both wrong.

The hottest grill is actually Kennen.

You see, a bolt of lightning reaches temperatures of 53,540 degrees F.

But if we had Leona unleash the full power of the sun, we're talking 27 million degrees F.

So she's back on top right? Actually no. A type II supernova clocks in at about 180 billion degrees F, and Aurelion Sol is more than capable of reaching those temperatures in his palm or wherever he decides to crush/end stars.

But we're not done yet.

The hottest temperature possible is the Planck temperature. This unattainable heat clocks in at about 100 million million million million million degrees. Only the instant after the Big Bang was temperatures this high.

A popular theory among scientists is that this universe was formed by the collapse of the one before it, once everything was sucked into a supermassive black hole. The singularity finally went critical. Reality got hit with the most powerful reset button in existence.

TL;DR Dark Star Thresh Veigar is the hottest grill :^)

(Sorry Dark Star Thresh, you may be able to eventually reach this point, but Veigar can access this kind of heat on demand.)