To me Alec Guiness as George Smiley is the gold standard. Smiley's People and Tinker, Tailor, Soldier, Spy. The Little Drummer Girl is good too.
If you haven't seen it get a copy of The Girl with the Dragon Tattoo and the sequels. The Swedish ones, not the toned down, too pretty, and bland, mass market one.
Wallander is good, but there you have to choose between a couple very different actors, Kenneth Branaugh or Krister Henriksson. The first is easier to find, but I prefer the second.
Cracker is harder to like, but better than most.
Inspector Lewis is better than the Midsomer or Brokenwood series.
Prime Witness with Helen Mirrin is very good.
Heat of the Sun was one of the very best Masterpiece Theater productions. It's one I would dearly love to see another dozen shows to have followed. (They didn't).
City of Vice is 17th century drama around the start of the Fieldings who were the foundation of Scotland Yard. It's not for the squeamish.
The Tunnel is good. So is Comrade Detective a Russian series.
Colonel March of Scotland Yard is an old series with Boris Karloff in the lead.
The Worricker Trilogy is another I wish they'd made more of.
I've just started Inspector Manara, an Italian series and I'm still on the fence with this one.
Don't forget Life on Mars, a time travel story line.
And for a strange American/British series with a comedy theme there's always Keen Eddie.