but he's the chief protagonist of Ninja Gaiden and that's where he appeared first. Luigi is in Super Smash Bros. and Mario Kart and Cloud Strife is in Ehrgeiz but those are not the most prominent roles for either character.
Depends on how you define it. I'd consider the prominence of the game as well as the prominence of the character in said game, and by that metric, Luigi's Mansion obviously loses to Super Mario Bros.
It would be interesting to do one where the answer is the first game they ever appeared in. So Mario would actually be donkey kong, I would imagine Luigi first showed up in Super Mario bros. you could have Diddy Kong who I think showed up in donkey kong country for Super Nintendo
The thing about these video game quizzes are that I've only played a few of the games and even though I know video games very well I blank on certain characters like I wouldn't be able to get Guitar Hero or Bioshock despite being very familiar with those games