With all due respect to Joel Fabiani and Rosemary Nicolls and their characters, Department S will be forever associated with Peter Wyngarde's Jason King.
Most people remember him as this camp, flamboyant and debonair womaniser cum detective in the mould of Austin Powers but that will do a disservice to the character: He's far more nuanced than that.
Jason King is lazy (he often lets Stewart fight all the bad guys and only chips in at the end), he is egotistical (his appreciation of people is based on whether they've read his novels or not), a lot of his detective work is speculation without facts to back them up and he sulks whenever Annabelle is right...and she often is. He's clearly a man having a mid-life crisis and drink drives but.......Jason King is brilliant. If Wyngarde had played him purely as a dashing hero, it wouldn't have worked but he shows King often as a paper tiger, led by his libido, love of finery and prone to grandstanding (and it gets in the way of his detective work at times) but he has some of the best lines and put downs in TV history. And by not playing him as whiter-than-white, the chemistry and interactions between the three lead characters is all the better for it.
Watching it again on DVD recently, you get to see just how much depth Wyngarde put into Jason King.
