That's a tough one.....I'd say September and not just because it contains my fav episode (Murder By The Book).
February comes close but is spoiled by Mind Over Mayhem, my vote for the weakest 70s episode.
October is also pretty good but has a couple of less than classic guest killer performances (Milland and Albert)
No, September it is for me....although Vera Price's "LbutL" performance isn't anything special, it's more than made up for by the inclusion of the magnificent Vincent Price!
I'd say March was the weakest, though that's a relative term as there's no such thing as a bad Columbo!

good picks steve...

and headache.................oh!!! you never cease to amaze me!

i tend to agree even though one of my favorites is in september "etude in black"...just love John Cassavettes

YAY!!!!!!