The only series I read that no one else has mentioned is the Sunfire romance (http://www.reocities.com/brynahilde/sunfire/) series of ridiculously addictive historical teen romances. Each book had a beautiful girl on the cover, with pictures of the two guys she would have to choose between next to her. You could almost always tell which guy she would end up with because it would be the one she was standing with in those little pictures. I loved historical fiction (Anne Shirley and Laura Ingalls being two of my favorites), so these were perfect for a preteen, absurd as they were. I remember especially being into Laura (http://www.reocities.com/brynahilde/sunfire/laura.html), just for the cover -- look at that adorable sailor suit and her cute boyfriends!

(I didn't want to duplicate everyone else's responses, but this is reminding me that I really do need to do a self-appointed project and sit down to write out my favorite books age 5-12 and what I liked most about them, since I'm considering writing some children's fiction and I want to remember what made a book great for me at that age.)