That one is easy for me: Paul Galesko. He was arrogant, shot a likeable and innocent man, then calls Columbo "not very bright" right before Columbo nails him. Great scene at the end.

TRY AND CATCH ME and ANY OLD PORT both have killings right out of CASK OF AMONTILLADO (of course, in ANY OLD PORT there's an actual connection to it). And whether Edmund killed Phyllis or not, that makes them both pretty horrifying.

Riley Greenleaf is the type that I would have liked to see smacked around. Many many murderers are rude to Columbo but Greenleaf is consistently rude.
Dang, my mind is going and I can't think of that actors name. But he played the writer in season one and was totally contemptible, not only for killing his writing partner but poor Mrs. Lasanka.

Interestingly some believe that that actor was fairly contemptible in real life. When his son achieved fame in the Partridge Family the actor said, "I just hope I am in my son's will." Or words to that affect. Dummy didn't even say it privately.