What is the ruling on one who says "the disbelievers of Quraysh are the most severe people in disbelief"? And does he commit an act of disbelief if they were not so?

The questioner is suffering from obsessive thoughts (waswasa), and this question is influenced by them. Stating that a particular group of disbelievers is more intense in their disbelief does not cause the one who states it to become a disbeliever; rather, it is a question about right and wrong. As for the issue itself, ruling on it requires a specific text. Although the disbelievers of Quraish were distinguished by their harm to the Prophet, peace and blessings be upon him, which is why Sa'd ibn Mu'adh said: "O Allah, if You have preserved any of the war with Quraish, then preserve me for it, for there are no people dearer to me to fight against for Your sake than people who harmed Your Prophet, disbelieved him, and expelled him." It should be noted that the disbelievers of Quraish were not all at the same level of disbelief. Among them was Abu Jahl, and among them was Abu Talib, who will receive the lightest punishment among the people of Hellfire, as the Prophet, peace and blessings be upon him, said: "The person who will receive the lightest punishment among the people of Hellfire is Abu Talib; he will wear two sandals from which his brain will boil."