Well, in the manga, Yuji has a lot of growth and potential. However, Mahito is no pushover. His cursed spirit nature and devious tactics make him a force to be reckoned with. So, it's hard to say for sure who's stronger.
It's a complex situation. Different storylines and villains have targeted the Avengers, but no one specific character can be said to have killed them all outright.
Kang's demise in comics is a complex matter. It could be a result of a team-up of heroes, or it might be due to an unexpected turn of events involving multiple characters and their actions.