The Conspirators is my least favourite of the pre-1980's episodes.. I've only seen it once and found it deeply unsatisfying. I didn't like the actor playing the murderer or his portrayal. I also felt the actual character of the murderer bereft of sympathy and quite simply a nasty piece of work. Even Columbo seems tired and lonely in this glum episode...

As much as I like Clive Reville, to me it has a lot of hills and valleys. This might be a ridiculous complaint, but one of mine is the title, since it's nearly the only title of the whole original series that doesn't try to be colorful at all, so it sticks out for that reason.
I do like things like the comical pub scene with the limericks (I'm pretty sure it's the first place I ever heard that "What a wonderful bird is the pelican" one).
Some people have said they really prefer Jeanette Nolan in THE CONSPIRATORS to Jeanette Nolan as uptight Mrs. Peck in DOUBLE SHOCK, and that's understandable, but to me it's the other way around.

Dead Weight, A Matter of Honor, A Case of Immunity and Last Salute to the Commodore are my least favourites.

Agreed with "A Case of Immunity", also I would add "The Most Dangerous Match." Threw the guy in the large recycle machine like a meat grinder?