Spies have been infiltrating television shows around the world for many decades now-our spies, their spies, bad spies and good spies. Do you have any favorites? Just whisper in my ear, and it’ll be our little secret.
The Avengers, of course.
Danger Man/Secret Agent.
The Prisoner.
The Man from UNCLE.
Get Smart.
Mission: Impossible.
The Americans.
One I saw recently on board an LOT airliner: Illegals, an excellent Polish series filmed in four different languages (with English subtitles).
24, if we can consider that a secret agent show.
I liked the smooth patter and no nonsense roughness of I Spy.
The Prisoner.
There was one in the '60s that I really liked: Blue Light, where Robert Goulet was an American double agent inside Nazi Germany. Unfortunately, it lasted only 13 episodes, IIRC. 
I should mention The Wild, Wild West as well.
We still wonder how it happened, because we usually encouraged our grade school aged kids to watch intelligent, challenging television, buuuut… as soon as ALIAS came on, the kids would jump in our bed and giggle whenever Sydney Bristow dropkicked a bad guy.
Recently rewatched it on Netflix, and it holds up. Jennifer Garner and Bradley Cooper are great as friends, and Victor Garber makes a wonderful “Spy Dad”.
Later, my kids returned the favor and got me watching BURN NOTICE.
Which I like mostly for the voiceover. (As someone said of the book
Beat the Reaper by Josh Bazell, if you’re intrigued by a hit man who, as he’s attacked by a gangster, pauses to tell the reader about how the elbow is constructed and how he’s about to deconstruct it.)
I loved both Reilly: Ace of Spies and Foyle’s War. Most of the others on the Top 100 list I have yet to see (except for the ones I’ve already listed).
I like British espionage TV shows such as The Sandbaggers, Callan and MI-5.
Chuck of course! Completely silly nerdy nonsense that made me smile consistently for five years.
The answer to this question is “Human Target”.
I have to put in a word for the 1979 TV series of John le Carré’s Tinker Tailor Soldier Spy, with Alec Guinness.
It’s rated #7 on IMDb’s top 100 list, and the sequel Smiley’s People #6.
Realistic, slow, unglamorous, tense.
Others I like have been mentioned or are on the list. One not mentioned is Killing Eve.
Definitely The Sandbaggers. The most realistic spy show ever. It ignored the cliches of the genre and never made things easy for the characters, who many times failed in their mission. Roy Marsden was spectacularly good.
Get Smart
A similar British show starring David Jason, Top secret life of Edgar Briggs.
Deutschland 83, 86 and 89 form an impressive and always watchable story arc.
And big votes for Tinker, Tailor and Smiley’s People
Having heard about this show my whole life, I purchased the Blu-ray set containing every episode and watched all of them. A great series, though there were a couple episodes that I thought were stinkers.
Thanks to this thread and the fact I’m on holiday through Thursday next week (China’s “Golden Week” holiday), I’m binge-watching the series now on YouTube.