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.