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.
Toge Inumaki is important too. His special cursed speech ability makes him stand out. Also, the principal of the school, Masamichi Yaga, is likely to be important as he is in a position of authority and has an influence on Gojo and the other students. There could be other characters like the members of different clans that also play roles in the complex web of the story.