This thread has brought back so many memories! Names I haven't thought about in years - Trixie Belden, Cherry Ames, Harriet, Ramona.
My favorite series as a kid was the Betsy & Tacy series by Maud Hart Lovelace. It was about two little girls and their families in turn-of-the-century Minnesota. What I liked as a kid was that as the girls got older, the reading level of the books advanced as well. Hardly anyone I've mentioned them to has heard of them, so when they were mentioned in "You've Got Mail" (by Meg Ryan as a children's bookstore owner) I almost fell out of my chair.
The other book I loved was "The Westing Game" by Ellen Raskin. It was the first "mystery" book I ever read, a puzzle mystery at that, and I was hooked. I still go back and re-read it every few years and enjoy it just as much.