I agree with David. It is difficult to explain, but in my opinion there is a freshness to the first season episodes (and to Falk's portrayal of Columbo in them) that was never quite matched later on - though the later seasons have many great episodes as well.

That is a tough call! I have favorites in every season, but thinking back to what really inspired me when I initially saw the seasons, I would have to say it was the exotic flair of the 5th season that really captured my imagination. Overall, 3rd and 4th are probably my faves (3rd favoring slightly higher), but look at all there was to learn about the world, in the 5th season, for those of us who weren't well-traveled...

A Case Of Immunity - diplomats!
Identity Crisis - spies!
A Matter Of Honor - arriba!
Now You See Him - ...Nazis?? Ok, sure, why not!

It was pretty exciting, to my inexperienced young mind. :)

Yes, season five found the Columbo films changing slightly. In many cases the shows seemed a little more experimental, and perhaps a little less clear cut, with regards to the plot, and the motives etc. 'A Case Of Immunity', and especially 'Identity Crisis', seem a little less obvious with regards to motive, and even to a point, 'A Matter Of Honor' which isn't really simply about the murderer being wronged, in some way. 'Last Salute To The Commodore' also is by far the show's biggest departure, of the seventies series, anyway.

What's great about season 3 and 4 also is perhaps the fact that some of the murders possess a degree of sympathy, perhaps, on behalf of the viewers. The Donald Pleasence, Johnny Cash, and Patrick Mcgoohan characters. especially, unlike many of the murders from the early seasons, may not have been truly bad people.

I believe too that season three may have had the better films than season 4, excepting perhaps the more mediocre 'Lovely But Lethal', and 'Mind Over Mayhem'. Season 4 is great, but i think the padding on 'An Exercise In Fatality' does blunt its effectiveness somewhat. I believe The Columbo Phile makes a very valid point here, on this occasion.