Two British series I love are “Midsomer Murders” and “A Touch of Frost.” I’ll add “Poirot” to the list as well.
A lot of great series have been mentioned above, but very few that predate the 1990s.
“Danger: UXB” was a superb '70s series about wartime Britain. (More of a mini-series, actually, but who cares?)
“Fawlty Towers” is one of the funniest sitcoms ever. Only a dozen episodes were produced in the '70s, but they’re classics.
“The Avengers,” “Danger Man”/“Secret Agent,” “The Prisoner,” “The Saint,” all from the 1960s.
“The Persuaders,” from the early '70s, which unfortunately ran for only one season.
“The Fugitive,” “The Man from UNCLE,” “The Wild, Wild West,” “Batman,” “Mission: Impossible,” “Combat!” all from the 1960s. I’d add “The Gallant Men,” but it only ran for one season and I don’t know if it’s available on DVD. “The Time Tunnel,” “12 O’Clock High,” and “The Rat Patrol” I would also buy if I could.
“Get Smart,” “Hogan’s Heroes,” “McHale’s Navy,” “Hennesy,” “Gilligan’s Island,” “The Beverly Hillbillies,” “Green Acres,” “The Andy Griffith Show,” “Gomer Pyle, USMC,” and “The Dick van Dyke Show,” all great comedies from the 1960s.
“The Jack Benny Program,” from the '50s.
The original “The Untouchables,” with Robert Stack as Eliot Ness (1959–63).
The first five seasons of “Bewitched” and the first three seasons of “That Girl.”
The original “Alfred Hitchcock Presents,” “The Outer Limits,” and “The Twilight Zone.” All later versions are pale imitations. Plus the British version of “Roald Dahl’s Tales of the Unexpected.”
The first three seasons of “Hill Street Blues.” All of “Lou Grant,” “St. Elsewhere,” and “LA Law.”
“Crime Story” from the '80s, though it went downhill rapidly after the first season. The same for “Twin Peaks.”
The first three seasons of “Miami Vice.”
The original “Dallas” and “Dynasty,” though the latter is now pure camp.
All episodes of “Columbo.”
I also quite liked “Hart to Hart” in the early '80s. Robert Wagner and Stephanie Powers never looked better.