The "quality" thing is probably not the best factor to look at. Personally, and I know a lot of people here will disagree with me, but I think the original Super Mario Bros engine is terrible and just about any platformer has a better quality one. Point being, there's a lot of subjectivity to what has better "quality". I think it would be better to look at how comparable the fangame is to official games; even if the SMB engine is terrible, a game that is too similar to SMB might be something Nintendo would attack.