So many, many great TV series to choose from. Some personal favorites that come to mind, in no particular order, though doubtless there a dozen more I'm forgetting ...
= Tinker, Tailor, Soldier, Spy, 1979, (starring Sir Alec Guinness' brilliant performance0
= Smiley's People, 1982, excellent sequel to Tinker, Tailor, Soldier, Spy
= I, Claudius, 1976
= Breaking Bad, 2008-2013
= Happy Valley, 2014-2021, though I've only seen the first 3 seasons
= Homeland, 2011-2020
= The Expanse, 2015 to present
= X-Files, 1993-2018
= Gamer of Thrones, 2011-2019
= The Wire, 2002-2008
= The Sopranos, 1999-2007
= Rome, 2005-2007
= Narcos and Narcos: Mexico, 2015-2021
= Peaky Blinders, 2013-?