This is not necessarily in order of preference:

(1) Columbo
(2) Rockford Files
(3) Kojak
(4) Mannix
(5) Cannon

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!

1. Columbo

2. Dragnet
3. Hill Street Blues
4. Adam 12
5. Car 54 Where Are You?

1. Columbo
2. Ellery Queen
3. Dragnet
4. Kojak
5. Burke's Law

"Columbo"
"The Untouchables"
"The Naked City"
"Kojack"
"Dragnet"

and

"San Francisco Beat"
(a few scenes were filmed in front of my Grandparents house, and I loved to see views of my home town.)

Cool!! That must have been great to see your grandparents' house there......but sadly I don't know the show....who was in it?

Columbo-of course

NYPD Blue-until Jimmy Smits left

Mannix-always some really cool cars in that show

Barney Miller-remember Jack Soo after he ate the brownies

Law and Order

Barnaby Jones-it isn't top five, but the scenes of him in his lab sure were different

Sheesh...I'm going to have to Google that one. I was a kid, and it was close to 50 years ago!

Jack Soo!! My father loved him!! He was a Kick!!...Jack Soo and my father!!