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.
In the comics, Negan is responsible for the deaths of multiple characters. For instance, he brutally kills Glenn and Abraham, which had a huge impact on the story.
Thanos kills a bunch of characters in the comics. He's notorious for eliminating powerful heroes and causing chaos. It's hard to list them all precisely, but it's a significant number.