NBC (ended 2003)
Hard to narrow it down to one, but my favorite episodes would include...
"Death Lends a Hand", Robert Culp at his best, and the ending is a hoot.
"Negative Reaction", Dick Van Dyke makes a great murderer, and the way Columbo gets him to imcriminate himself is classic.
"Any Port in the Storm", the best in the 'sympathic' murderer vein.
"Forgotten Lady", the finest 'twist' the producers pull in the entire run, the ending is truly moving and unforgettable.
"Columbo Goes To College" always comes to mind, I love how those college students mock and imitate Columbo throughout the episode.
My favorite "original" Columbo episode is "Exerice in Fatality"; Columbo trying to keep up with Robert Conrad's exercise routine is classic!
Any of the three with Jack Cassidy ("Murder by the Book," "Publish or Perish," and "Now You See Him"). "Now You See Him" is probably my favorite, but Cassidy was such a great Columbo villain. He could barely contain his contempt and disdain for Columbo, and his face when he knew Columbo had him trapped was always priceless.
Other favorites are "Etude in Black" with John Cassavettes, "Suitable for Framing" with Ross Martin, and "Negative Reaction" with Dick Van Dyke I'm not as much a fan of the newer, ABC episodes, but the original NBC movies are fantastic.
It's tough to pick out a fave but a few of the ones i enjoy watching the most are:
Murder by the Book - Jack Cassidy
Etude in Black - John Cassavettes
In Exercise in Fatality - Robert Conrad (and Gretchen Corbett of Rockford Files fame)
A Friend In Deed - Richard Kiley
Prescription Murder - Gene Barry
The Most Crucial Game - Robert Culp
Double Exposure - Robert Culp
and I always liked Blueprint for Murder
I'm sure there are a couple of others but these are the ones i thought of first