Pick a certain "prop" that a murderer uses...something that strikes you or enchants you or that you just like.

I would say Grace Wheeler's cigarette holder in Forgotten Lady. She uses it with such an eloquent manner.
Also Colonel Rumsford's cigar clipper in Dawn's Early Light. When he hands it to Columbo it is like Columbo has just been handed the Holy Grail!!!

Didn't Lee Grant hand Columbo the controls to her airplane?

(It's been awhile since I've seen it, so, not sure.)

One of the most outrageous props has got to be that pink spray used by Roger Stanford in "Short Fuse."

Why was Columbo spraying it all over himself.. wouldn't he be more cautious since he had never seen it before?

Oliver Brandt swings a mighty nasty umbrella in "The Bye-Bye Sky-High Whats-It Hubba-Bubba Murder Most Foul And Poultry" episode!

Another prop, this one necessary considering the script, is the hearing aid for Emmett Clayton in "The Most Dangerous Match."

2 Dobermans

or the block of ice from the ice cream truck

Let's not forget Harold Van Wyck's fancy-shmancy watch in Playback. And also Ward Fowler's "hi-tech" $3000 vcr. Or even that luscious carrot juice that Milo Janus loved.

The Maestro wielded his baton with memorable style in "Etude In Black".

The electrix mixer from "Double Shock" would make a cool Columbo souvenir. As would the smashed watch from "Candidate For Crime".

Or Galesko's quaint, fold-up camera in "Negative Reaction".

Turning to the "new" episodes, for unusual props it's hard to beat the guillotine in Columbo Goes To The You-know-what.

The painting of Columbo in "Murder: A Self Portrait" would be a sort of ultimate bit of Columbo memorabilia.

Also, a really scary prop, used by McGoohan in a memorably scary way, was the canula or arterial drainage tube (it was sort of like a medical-looking fireplace poker) in "Ashes To Ashes". CLANNGGGG!!!!

Ted!! how could I have forgotten about the Grand Maestro??.....But you are that for me!
How about Tommy Brown's guitar in Swan Song...or the ashtray/buckle in Old Fashioned Murder..or the electric blanket in Suitable for Framing...or the ring on Brimmer's finger in Death Lends a Hand.............wow!! the list is endless!

I erred in forgetting to mention Roger Stanford's camera in "Short Fuse." Nice touch to an already complex character.