My first mysteries were the Happy Hollisters series.
Anyone else remember Pete, Pam, Ricky, Holly and Sue?
As to adult fare, Miss Marple stories by Agatha Christie (pinging our own Miss M!) and I do consider Sherlock Holmes to be mysteries.
I need to ponder something to recommend that might be different.
(I have lots of Perry Mason paperbacks I got from my grandmother...I read them all when I first acquired them years ago...)
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.
We're still reading the Happy Hollisters at this house. And the Boxcar Children too.