My vote goes to Checkmate, which ran for 70 episodes from 1960-62. It was created by the great Eric Ambler and starred Sebastian Cabot as an Oxford professor turned investigator. In manner and appearance Cabot, a highly intelligent character actor who is now sadly forgotten, resembles Stephen Fry. The series featured ingenious stories with clever plot twists and clever dialogue.
That was one of my dad’s favorite shows!
Jeremy Brett in Sherlock Holmes
James Garner in The Rockford Files
Leo McKern in Rumpole of the Bailey, not sure if it really qualifies
All three of them are simply not to be missed.
Magnum. Loved it when it was first on, and I still find it very watchable.
When did it come into effect?
This is what I came in to mention. I really enjoyed David Janssen’s world-weariness.
Here’s the thing: I have to nominate Monk as well.
When will I thank you?
You’ll thank me later. 
[smiles and nods to everyone else]
He’s the guy.
[/smiles and nods to everyone else]
(Unless I’m wrong – which, y’know, I’m not.)
Another vote for Rockford.
Jonathan Creek, starring Alan Davies’s hair.
Anyone else watch Simon & Simon? Always enjoyed it.
Rockford of course and the Brett Sherlock Homles
Another vote for Sherlock Holmes with Jeremy Brett.
The Saint deserves a mention. I like the play on his reputation and not exactly trusted by the law.
It’s too bad “Detective Conan” A.K.A. “Case Closed” never caught on in the states. Because it’s anime the only place it could air the audience prefers a show about a teenager shocking everyone with the size of his sword. But it probably wouldn’t be accepted on a channel like A&E either because it’s cartoon about a kid in shorts. It does a good job of presenting interesting murder puzzles and solving them with deductive logic.
I like the show. It’s a nice break from all the Inspector Whoever imports.
Umm…
Wipe. Wipe! Wipe!
Simon & Simon
Magnum
Moonlighting
I suspect since I like Simon & Simon I would like Rockford files, they seem similar as far as setting and style.
It only got a season, but I really like(d) Terriers. It was the only P.I. show I ever actually enjoyed. Most are boring to me.
I haven’t seen most of the series mentioned here. I did watch the first season of Veronica mars though, and thought it a very refreshing take on noir for the modern age. I’ll throw it up for consideration
Johnny Staccato starring John Cassavetes had some amazing episodes and a lot of good music. It’s impressive how shows like this and Peter Gunn fit so much atmosphere, plot, and music into just 30 minutes.