The ending of The Conjuring: Last Rites has the Warrens defeat the demon by praying together, which breaks the mirror's power and frees both Judy and the Smurl family. The Smurls get years of peace in their home, the mirror goes back to the Warrens' museum, and the final scene is Judy's wedding, where Ed gives her away and Lorraine sees a warm vision of the family's future.
What does that mean? In my opinion, the film is built as a farewell to Ed and Lorraine rather than a setup for a direct follow up. Lorraine saving Judy a second time, after the 1964 birth, closes the circle the prologue opened, and the wedding gives the couple a quiet exit that the earlier films never offered them. It could be argued that this is the safest ending a long running saga could choose, since it trades a final twist for emotional payoff, but it suits characters who have spent four main films defending other people's homes.
Is Last Rites the end of the saga? Not entirely. Warner Bros. has dated conjuring first communion for September 10, 2027, according to TheWrap, so the series carries on past this film.
Is it worth watching, and is it the scariest? As a fan of the monster side of this series, I would say it is worth watching if you have followed the Warrens, because the ending depends on their history with Judy. Whether it is the scariest is a matter of taste, and in my opinion the first film still has the tighter scares, while Last Rites is the more emotional one.