What was your first favorite Mystery series?

What was your first favorite Mystery series to read? My first, that I remember, was the Bobbsey Twins series of mystery books. I loved these books and one of my favorite was The Bobbsey Twins: The Secret at the Seashore. Next came Nancy Drew books and I was hooked on mysteries for ever after that. My sister and I had  a lot of these books and when my daughter got old enough I bought her some of them and then I bought a few for my Granddaughter too. My brothers were reading The Hardy Boys series and yes I admit I read some of them also. One of my favorite Nancy Drew books was The Hidden Staircase as well as The Secret of the Old Clock. Truthfully I loved all of the books in both of these series and it is hard to choose a favorite. What book got you hooked on mysteries? What book got you hooked on reading, whether it was a mystery or not? I even, today like to reread these series for nostalgia sake. They are good reads whether you are 10 or 110. Good Reading!