I used to love the Babysitters Clubseries. My best friend at school had the entire series (up to date in the 90s - I don't know if she still has them) and I was envious of her collection. Her books had the bricks on the side and they looked so cool next to each other. She did kindly lend me a few books. I wanted to babysit after that but I didn't have many chances as a teenager.
I enjoyed the Sweet Valley series as well. I enjoyed the TV series. I did think though that Elizabeth and Jessica had no flaws in the Twins series whereas they were slightly (but not much) more realistic in the High series.