funniest Columbo scene

You're right, it was unscripted. Here's what iMDB says:

When Columbo walks down a steep, grassy hill to inspect the wreckage of the Jaguar, he loses his footing and stumbles, taking a big fall and landing on his back. This wasn't staged; it was Peter Falk, not a stuntman.

Which, I think, makes his remark even funnier. "But I'll tell ya, it was the quickest way down."

One of the funniest scripted scenes was in Negative Reaction with guest star Dick van Dyke

Columbo visits a soup kitchen to try to find the homeless man who was in the junkyard when Paul Galesko (van Dyke) confronted and shot the supposed kidnapper of his wife.

The nun who runs the soup kitchen takes Columbo for a homeless man and offers him a hot meal and a new coat.

Very funny scene!