I enjoyed watching Columbo interact with the affable senatorial candidate played by Jackie Cooper, and also the affable Congressman played by an excellent actor who is unknown to me, in "Agenda for Murder".

So my answer to your question is politician, and I wish they had done more of those. Every once in a while I see one of our fine representatives on TV and I think, I'd love to see the Lieutenant square off against that guy.

I love that Jackie Cooper episode, too! The Nelson Hayward character in that episode seems to supremely represent two of the more contemptible things that many real-life politicians have been guilty of- unfaithfulness in marriage and being a hypocrite (Nelson campaigning against crime, yet being directly involved in it). I also love how his campaign manager, the victim, is a completely unlikable and revolting character, too.
I've only briefly seen parts of "Agenda For Murder", but I think I know who you're referring to- the guy who Patrick McGoohan's character is sort of in cahoots with. I agree that politicians make great adversaries, especially since they have so much to lose when Columbo catches them, so are naturally very contentious and defensive (sometimes to hilarious effect).