Is the rank of the righteous (Siddiqin) exclusive to the early generations (Salaf) due to the Hadith: "The best of generations is my generation, then those who follow them, then those who follow them"? And is it the highest of ranks and the opinion of the majority, and which scholars affirmed it?

We have not come across any statement from scholars indicating that the status of Siddiqiyyah (truthfulness/righteousness) is exclusive to the early Muslims (Salaf). Rather, it is a rank attained by anyone who embodies its characteristics. It is the highest rank among creation after Prophethood and Messengership. Allah associates them with the prophets in His Book, in His saying: "And whoever obeys Allah and the Messenger - those will be with the ones upon whom Allah has bestowed favor of the prophets, the truthful, the martyrs and the righteous. And excellent are those as companions." They are the Rabbaniyoon (devoted worshippers and teachers of divine knowledge) and those firmly rooted in knowledge, and they remain upon the truth. Whoever worships Allah by vexing His enemy has taken a significant share of Siddiqiyyah. And the likeness of the Ummah is such that it is not known whether its beginning is better or its end.