Jack Cassidy - my favorite was in Murder by the Book
Patrick McGoohan - my favorite was in Identity Crisis
Robert Culp - my favorite was in Death Lends a Hand

"Mr. Cool personified Robert Culp."
I was a kid in the 70s, so I've always known who Robert Culp was, in the, "Oh, it's that guy!" way–but it wasn't until a few years ago when I found Columbo & started watching that I realized what an awesome cool smokeshow 70s Robert Culp was!

I gotta go with smarm master supreme Jack Cassidy and Mr. Cool personified Robert Culp.
You've seen Guy Standeven in something because the man was in everything.

You took my two favorites. Cassidy was always so smooth. He and Falk were always fun to watch together.
Culp was always angry while it all just rolled off Columbo.
I enjoyed most of the killers, men and women because most of them always thought they were so much smarter than Columbo and they were so condescending because they thought he wasn't too bright. They had no idea that underneath the befuddled facade, he was the smartest guy in the room.
While Johnny Cash only appeared once, he is my favorite killer. He and Falk were terrific together.
You asked a pretty question; I've given you the ugly answer.
Fasten Your Seatbelts….
It's Going To Be A Bumpy Night!

I choose
Johnny Cash
! He wasn't a
multi-episode
killer like everyone mentioned above, but he was the only killer guilty of multiple-victims at once. (There are a few other multi-victim episodes though… the experts on the panel know which ones I'm talking about).
Johnny Cash, in the woods, with the airplane.
Final answer.

Patrick McGoohan.
"I'll make him an offer he can't refuse" - Don Vito Corleone - The Godfather

That is one of my favorite Columbo scenes. I love when he irritates a suspect to the point of total distraction.

Yeah, that's great. At one point, Culp's Paul Hanlon gets so rattled that when he looks through his binoculars, he's not even looking in the right direction. "They're on the 10-yard line", says Columbo helpfully.

The final scene of "Most Crucial Game" when Culp loses it is a gem.

I like Culp. He's not a cool customer like the other two. His characters bring a full range of emotions. I'll take Cassidy second. He's a little more calculating and evil.

Culp was the murderer in "Double Exposure", "Death Lends a Hand" & "The Most Crucial Game. He also played the father of one of the murderers in "Columbo Goes to College."

How many was Culp in?
I only know of two.
I admit that I just don't know much at all - Heeeyyy117

"You call that a lining?"

and he is very good in Agenda for Murder as well.
"What the hell am I gonna do? What the hell am I gonna do?" I find myself saying that over and over. (Oscar Finch tells Columbo that's what the murdered guy said on the phone before being killed.) It's more the sing-song delivery than the actual words that get me.

Patrick mcgoohan. especially like him in identity Crisis, and he is very good in Agenda for Murder as well.

Yeah, Cassidy (or at least, his Columbo characters) had a smarminess that could be a bit off-putting. Cassidy had a thin-lipped smile that looked incredibly fake and insincere. (That may have been probably was his natural smile, but it just looked forced.)
Culp was always my favorite I cant say why, exactly theres just something about him. (
Love
that audacious bright yellow slim-fit suede jacket he wore in Double Exposure! Only Culp / Dr. Bart Keppel could ever pull that off.) (And remember, its
Doctor
Keppel!
)
McGoohan is great, too, of course, but Culp is still my fave. McGoohans distinctive voice/accent, and just his general manner, made him come across as a bit distant and unapproachable. (That's not necessarily a bad thing, though. Sometimes it's exactly what you want for the character in question.)
So if I put my favorite Columbo multi-murderers in order, it would be:

1. Robert Culp
2. Patrick McGoohan
3. Jack Cassidy

there's an interesting 'Mysteries & Scandals' episode on Cassidy posted on Youtube that tells his bizarre Hollywood history, in cas you're interested

He seems somehow pretentious to me. His episodes aren't bad but like I said, I no longer record them.
I love that Robert Culp gives Columbo the business EVERY SINGLE TIME. When he's upset; he freely shows it.
I like that McGoohan plays mind games with him.
Although not a fan of "Star Trek", I liked both episodes with William Shatner.

I can see how Cassidy might grate on a person's nerves, though I do like him.

Patrick McGoohan and Robert Culp. Not a fan of Jack Cassidy; I've stopped recording his episodes. Not sure what it is but there is just something about him I don't like.

Though Culp is my personal favorite, McGoohan was certainly one of the best.

I have to second Patrick McGoohan.
He was so deviously wonderful in his appearances. Also, McGoohan directed 3 of the 4. He was very talented.

I kind of agree and disagree - in that McGoohan's performances are all his core persona (condescending & pretentious) whether a politician or an administrator. Still totally love his contribution, even as a frequent director. A friend told me Falk and McGoohan became quite close friends after the initial pairing, and McGoohan's ongoing involvement with the series (old & new) was at Falk's request.

Actually, he had 3 characters, and yes, they were all different. I enjoyed Culp and Cassidy, but I don't believe their characters had the range of McGoohan, he even looked different in each episode.