I would recommend "Negative Reaction." A lot of good clues, good tension between Columbo and the murderer, and that unforgettable cemetery scene which helps to define Columbo's character.

Agreed..."Negative Reaction". Has it all, great chemistry, just enough humor (not too much),
Columbo is
"modest and mildly eccentric" to the right
degree, a really clever set-up by Galesko making
the case seem uncrackable,
great acting by the victim (who often ends
up as a cipher in the different stories)
Antoinette Bower ("what is this Paul, some
newly found masculinity?") and great directing
and camera work. A "10"!