Neptunia Rebirth 2 and 3 plus Neptunia VII are considered the mainline canon games in the series.
Rebirth 1 is a game that re-tells the first game's story with side characters from Rebirth 3 interfering with events, changing what happens.

The rebirth games have only been available in English on Vita and PC with the exception of Rebirth 1 getting another remaster in Neptunia Reverse on PS5. Nintendo of America / Europe blocked a Switch release of Rebirth 1,2,3 and now you have to import them individually from Asia.