All the clubs on the second to fourth positions are threat to Napoli and don't also forget that Inter know how to stay strong for so long, and they are always among the title contenders from season to season. I know that Fiorentina and Atalanta are fully in form but it's for them to continue in this great form till the end is what no one knows if it will be possible or not. As for Lazio, I don't see them lasting long on top of the table. This season is not easy for Napoli like last two season that they won the title for the first time.
You are right, friend, that the teams ranked 2 to 5 are the teams that can threaten Napoli at the top of the Serie A standings. Because these teams (2-5) have the same points and all these teams only have a difference of 1 point with Napoli. So with this, Napoli is currently not in a safe condition, because if Napoli experiences a draw or lose, it is certain that one of the teams ranked 2-5 will immediately take over the top of the Serie A standings. Therefore, for now, Napoli really has to be consistent in every match by getting winning results, because if not, Napoli has the potential to be moved from the top of the Serie A standings.
Apart from that, talking about Inter, it can be said that this team is a very great team, because the proof is that in the previous season Inter was able to win the Scudetto. So if you say that Inter is Napoli's rival, I think it is true. Although this season, in my opinion, Inter's performance has decreased slightly. But still Inter will be the biggest threat to Napoli. Apart from that, I don't think we can underestimate Atalanta. Because Atalanta is currently a team that is really taken into account in Serie A. Because in the previous season Atalanta also managed to become champion in UEL. This is strong evidence that Atalanta really has a very good performance at the moment. Therefore, it looks like the competition in Serie A this season will be tougher than in the previous season.