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!