Camille at Book Moot recently posted about Nancy Drew and the Hardy Boys. I loved Nancy Drew but was annoyed by Frank & Joe. As a series reader myself, I agree with her statement that series readers become life long readers.
Other series I loved, in no particular order:
The Happy Hollisters by Jerry West
Katie Rose by Lenora Mattingly Weber
The Outdoor Girls of Deepdale by Laura Lee Hope (I also liked other pre-1920 girl series, but it was really hard to track them down so I tended to read only one in series such as The Red Cross Girls.)
Mary Rose at Boarding School by Mary Mabel Wirries (wow, I cannot find anything online about this series!)
Yes, I had a lot of hand-me-down, found in attics, or cheap at garage sale reading.
So what series books did you read?