Nancy Drew. There are approximately 8 zillion different bindings, but mine were the yellow-backed ones with full-color covers where Nancy has kind of a Marlo Thomas "That Girl" flip going on.
Nancy had it going ON. Yes, her mother was dead, but Nancy didn't dwell on that. Her dad was a hotshot lawyer who let her have her own blue convertible (changed to a hybrid in later books), she had a motherly figure in the form of housekeeper Hannah Gruen, she had two best friends, cousins Bess and George, she was recognized in every police department in the country, she apparently had neither a job or classes, and of course she had a boyfriend, although really, Ned Nickerson was pretty Ken doll-bland.
Growing up a girl in the 1970s was pretty OK in many ways. You really were encouraged to do anything a boy could do, and Nancy was a big part of that. I always thought of the Hardy Boys as her slightly slower cousins.
And today? She even has an iPad app, which I may just have to get.
Were you a Nancy fan? Do you remember any one book over the others? I also read other series like they were going out of style: Bobbsey Twins, Happy Hollisters, Dana Girls, Three Investigators, Trixie Belden...you name it. Any of those in your childhood bookshelf?