When I was a boy, I loved the Hardy Boys series. I was hoping my sons would like the Hardy Boys too, but then Harry Potter came along and the Hardy Boys couldn't compete.
As an adult I love Michael Connelly's books featuring homicide detective Hieronymus (Harry) Bosch. It's a somewhat unusual series, in that Bosch also shows up as a supporting or minor character in other Connelly books that star lawyer Mickey Haller. (A movie version of the Mickey Haller book "The Lincoln Lawyer" is being released later this month.)
Other series that I enjoy are Sue Grafton's alphabet mysteries starring private investigator Kinsey Millhone and James Patterson's Alex Cross series.
I am currently reading and enjoying Stieg Larsson's millennium trilogy. With any series, if at all possible, I try to read the books in order.