As far as I was concerned, there was only one man who personified the "English secret agent" and his name was "Steed." Bond was too clumsy and full of himself, Templar was too derivative, Smiley was too realistic, Cornelius was, well, at the other end of the spectrum from Smiley, and I was too young to really "get" Number 6.
Patrick McNee's John Steed struck just the right note. He had fun with the character (playing off of Diana Rigg, who wouldn't?), his timing was spot on, and while he didn't bring a great deal of physicality to the role, his bearing more than made up for it.
I won't lie: I'm not nearly as familiar with his other work (aside from The New Avengers, which was still loads of fun). Every time I'd see him in another film or show, my first thought was "oh, John Steed!", but he was always at the very least good, and frequently excellent, in everything I saw him do.
I hope he was happy up till the end, and even I know him primarily from one television show almost a half century ago, I absolutely loved him for it. James Bond wasn't fit to carry his umbrella.