Asides from the groups that I’ve mentioned already? I really enjoyed things like Sweet Valley High and The Babysitters Club when I was younger. Francine Pascal wrote some amazing books, although she seems to have gone off the boil with the latest SV update which is a bit of a shame.
Jasper Fforde and Janet Evanovich both write incredible funny books that you could read as standalone but also work well in their respective series’. Patrick Ness and Phillip Pullman have both written great trilogies for young adults as well – definitely worth a read if you’ve not already discovered them.
I tend to consume authors in one go if I find a new one I like, but there aren’t many series’ that I enjoy that are still going, unfortunately, so I guess I’ll have to just read the ones I love again. Such a hardship ;)