What makes a good series

I have just binge read the Outlander novels. It was an incredible commitment of time but well worth it. Kudos to Diana for her research and diligence. I love writers who take the time in the first book to set the stage for what's too come. Too many authors score big on book one and next thing you know there are sequels that just don't work. You need to do the groundwork and decide where you're going early on. Diana has done that beautifully.
I did find the hero Jaime to be a little too perfect and Superman but hey, I got the same complaints about my heroine Sunny. You can't please everyone.
To me, the secret to a good series is seeing how characters and relationships grow and evolve. I loved Jilly Cooper's Riders series. Outlander. Any others? Dungeons and Mercenaries is fun and very sexy. The Mitford series works. Any others? Let me know. I'm always up for a good series and writers from whom I can steal ideas on keeping my 6 books fresh.