The three definitely on my list are:

Forever (TV)
Highlander: The Series
Horatio Hornblower (books and miniseries)

Other than that, I don't know? There's so many to choose from. Ones that have run through my mind are X-Files, Quantum Leap (both of which cross over with just about anything), Star Trek TNG, Ripper Street, Doctor Who, Torchwood, Lucifer (TV)...