My classmates read "The Happy Hollisters," while I checked out "The Hardy Boys." I really liked "The Mark on the Door." The newer HB series, where one of the girlfriends gets blown up in a car bombing, doesn't quite suit me, though.
Mystery at Cabin Island was my favorite, followed closely by the Yellow Feather Mystery. The girl that gets blown up is Iola, Chet Mortons's sister and Joe Hardy's girlfriend. I never cared for the later ones, either.
A little off topic, but I just finished reading "Undaunted Courage" for the umpteenth time. Written by Stephen Ambrose, its a great account of Lewis and Clark and the Corps of Discovery..