I'm going to exclude Columbo from my list as it's always going to be number one. In no particular order.
- The Sweeney (70s British cop programme)
- Cops (Tragically unappreciated British cop programme from the 90s, cancelled after two series - a national disgrace)
- Homicide: Life on the street (The first few series were outstanding and novel, they tinkered with the format in later series which made it good but no longer great)
- NYPD Blue (The Caruso-Franz era was amazing. After Sipowicz stopped drinking the programme went toilet)
- The Shield (Just when you think there's no life left in the cop format - they come up with this.)
I am surprised more people didn't list Rockford, it is excellent and has a loyal fan base. The top 3 were all from Universal so we see the same directors, guest stars, etc.
I actually haven't seen Mannix for something like 30(!) years and I was a kid when it was on, but I recall it being a classy show. It was also made by Levinson and Link along with Bruce Geller who also made Mission: Impossible which is right at the top of my favorites.
I loved corpulent William Conrad who played Frank Cannon, the gourmet-bon vivant detective. Although it was made by Quinn Martin whose output was not as good as Levinson-Link-Geller, it was quite entertaining. I recall that he ended every episode by cooking up a gourmet meal!
I am still waiting for Rocford, Cannon and Mannix to appear on DVD, along with Mission: Impossible!