I'd say Nora Chandler (Anne Baxter) in Requiem for a falling star: she fainted when she came to know that her girl-assistant had died.
She looked rather convincingly (besides, at that point the audience thought that she wanted to kill that blackmailing star-gossip reporter).

I vote for Paul Galesko in "Negative Reaction".
After all, his wife was kidnapped and he was
shot by the perpertrator. He also went to
pieces when her body was being brought out
of the farmhouse and he tried to run on his
injured leg. Very convincing performance UNTIL
he stupidly asks his assistant, the scantily
clad Miss McGrath out to dinner, right in front
of Columbo at his wife's funeral!

Then he acts like a little child who doesn't get candy before a meal when she tells him she has other plans for tonight.

How about Viveca Scott (Vera Miles) in "Lovely But Lethal"? I picture her so sweetly saying "Murdered? How horrible." And she committed TWO murders.

i was going to say nora chandler, too.

also grace wheeler, although probably her innocent response was genuine, if she was supposed to not remember the act.

leslie williams's fainting was good, but it obviously didn't fool columbo. interesting how so many of the female characters in columbo collapsed with stress (or feigned collapsing from stress) following murders and needed physicians to prescribe them sedatives. sign of the times, i think...

This is partly "off-topic," but I always felt sorry (apart from the obvious reason) for Jean in "Requieum." (I'm not too familiar with Pippa Scott, but I know she was very good at playing sad characters.) When Nora tells her, "You don't REALLY BELIEVE him, do you?" it reminds me of someone with a crush, being given that "He's / she's out of your league" lecture. (Even though that wasn't the actual situation here, it somehow reminds me of that.) Which leads me to another thing. In spite of how "superficial" he's pictured as being, all through the story, does anyone believe that Jerry DID have feelings for Jean (along with using her)? In that scene mentioned earlier, where Nora sarcastically recites lines from that love letter, he gets visibly mad (about the only time he does), as though she's really struck a nerve.