I'd bet money that the most used pass is the Paris Museum Pass.
https://www.parismuseumpass.fr/t-en
It used to be a better value when it allowed unlimited visits to the attractions it covers - you're now limited to just one entry per each - and you'd have to (A) be interested in re-visiting attractions you've probably visited in the past, and (B) be able to do enough of those to justify the cost of the thing. It all comes down to math: the price/convenience of the pass vs combined price of individual tickets. Do be aware that even with the pass, some attractions, such as the Louvre, mandate advance reservations so read all segments of FAQs (bottom of the main page of the site) and the individual descriptions of each attraction for details. For instance, this for the Louvre:
https://www.parismuseumpass.fr/t-en/sites-a-paris/musÉe-du-louvre
This pass doesn't cover any public transport.