I find the murderers in the newer episodes to be more annoying. I think they were puposely written that way. It's hard to beat the two frat boys in "Columbo Goes to College".
As far as the older episodes, I think the writing focused more on making the murderers arrogant and boorish rather than just plain irritating. But I agree, Beth Chadwick was pretty annoying. Roddy McDowall was a bit irritating too.
Yep....Justin and Cooper from Columbo Goes to College are (and have been) my top pick forever. All that potential and money and famous (somewhat) fathers - at least justins was. They were both two big putzes.
William Shatner irritated me the most. In both episodes, "Butterfly in Shades of Grey" and "Fade into Murder." He is so arrogant and self-centered, I don't even watch those episodes.