9. His Resources Are Unlimited
Kang has bonded the chronal technology to his battle armour, allowing him permanent access to all timelines. Because of this, he can whip out any weapon from any time in an instant. No matter how strong Thor or the Hulk are, they simply cannot compete with Kang's anti-matter shields, EM amplifiers, neutrino warheads, paralysis rays, and molecule expanders. He can pull out as many weapons as he wants from any number of timelines, making him impossible to disarm.
He can even use the Avengers' weapons against them. Iron Man explained to his teammates that he once considered creating a weapon called the Dark Matter Accelerator that could turn a city inside out but abandoned the idea in case it got in the hands of a supervillain.
Kang ventured into the future to learn Iron Man ended up building this doomsday device after the world became too corrupt. The time-master then travelled to the present and used this same weapon to blackmail the Avengers to do his bidding.
Since he has seized a multitude of timelines, he has assembled armies from every era imaginable. The Avengers may be the world's best superheroes, but even they cannot defeat trillions of Kang's soldiers.