For my money, the overall best espionage show on TV was "Callan," a very hard-edged John Le Carre-style spy show starring Edward Woodward as a professional assassin working reluctantly for a branch of the British Government's intelligence services called 'the Section'. It ran from the late 60s into the early 70s. It was a near perfect mix of behind the scenes political maneuvering (the chess game) and action in the field.
The next best for me is "Spooks" (MI-5 here in the US), which also mines that same blend of dark intellectual gamesmanship with edge of your seat action. All 10 seasons are excellent!
I have only seen a few episodes of Callan, but I am a huge fan of Woodward's other show, The Equalizer.