There was one that I don’t know if I’ve run across in a long time. I remember reading it in the paper, and I remember it being The Far Side. Not sure if my memory is reliable, though.
Anyway…
A bunch of piglets are nursing at their mother’s side. One of them has popped up above the rest, and is saying: “Hey, I think I’m getting Orange Crush over here!!”
I still laugh thinking about it.
My all time favorite is one I got from a Far Side daily calendar several years ago and still have tacked to my bulletin board at work. It sort of encapsulates everything **The Far Side ** represents. (And it echoes my own feelings as well, though I’m not religious)
The drawing shows the interior of a room, with a window and a bookshelf, and on the wall is a cuckoo clock that is chiming except that in place of a cuckoo bird popping out, there is the Earth.
Caption: In God’s den.
On the same page, it recounts (paraphrased) "The farthest planet from the sun, Pluto was discovered in 1930 by the staff of the Lowell Observatory. Asked how they were able to spot an object so faint and so far away, the head of the observatory replied, “We squinted.”
My favorite was in the same section of TPOTFS, with several surgeons huddled around an operating table, and one holding up apparent table scraps for a begging dog…
One of my favorites: wolves in a cocktail party environment, one wolf is holding a sheep by its hind legs with its head in a bowl and says to another wolf, “Well, I’m addicted. Have you tried Carol’s sheep dip?”
Great, really great, but not the greatest. Glen Baxter comes to mind.
Another vote for CAT FUD.
A bunch of guys in Hell, and a dog. One guy is saying, “Well, I always assumed he was just a bad dog.”
Two guys and a dog on a lifeboat, all looking at a third guy; “Sorry, we need to eat someone to survive, and you drew the short straw.”
“Yes, they are all fools, gentleman. But the question remains; what kind of fools are they ?”
At a butcher shop : “Well I never thought about it, but I’d let the kid go for, oh, two fifty a pound.”
A janitor walks into a room with a giant robot; “Well, it looks like our test run got a little more interesting.”
The one about home surgery, “Stop squirming, Edna, I’ve almost got it !”
“They needed a scapegoat. They found Dwayne”
“Vampires ! Beware the vampires” - spoken by the only guy on the street with a reflection
“Look at the laboratory equipment, the scribbled calculations - curiosity killed these cats.”
Two explorers in a field, with a huge water buffalo a foot behind them. “Buffalo breath ? Buffalo breath ?! Need I remind you of your incessant little grunting noises ?!”
A pair of moose, the male fat and in front of a TV, the female dressed like a housewife holding a phone; “It’ the call of the wild.”
Larson is by far my favorite!
Second best is B. Kliban though some of his you can stare at a thousand times before you finally laugh.
Best ever?
No.
Better cartoonists abound.
Walt Kelly–Pogo
Will Eisner–The Spirit
Milton Caniff–Terry & The Pirates, amd his WW2 serviceman’s strip, Miss Lace.
The funny thing about “cow tools” was that I’m one of the few people I know who totally got that.
One guy in a hospital in a full body cast. The other guy, in safari garb, is recanting the story:
“So then I said to Jim, do you think that gorilla is dead or just asleep?”
Best ever? I’d not go that far.
My second favorite? Yes (after Watterson).
But even I won’t claim Watterson is the best ever. But he and Larson are the ones I’ve enjoyed the very most.
My favorite Far Side cartoon? Too numerous to name. For now I’ll mention the one of two kangaroos bouncing across the landscape, with one yelling at the other: “Just jump, fool! You don’t have to go 'Boing, boing, boing”’
Another robot one: kid walks into his metal shop teacher’s office and proudly announces, “My project is ready for grading, fuzzhead! Yeah I’m talking to you, squidbrain!” Behind the kid looms a 10 foot tall robot with razor sharp teeth.
In terms of artistic drawing ability, he wasn’t great, but his ideas transcended the artwork.
I’ve got several:
Dog Hell (dogs in Hell, obviously). Half are dressed as letter carriers, the other half are carrying pooper scoopers.
Two men sitting in Hell. One says to the other, “I really hate this place.”
An ant at the top of the anthill. As an anteater approaches, the ant screams, “ALERT! ALERT! ITS THE SUCKING DEATH!”
A scientist studying a slide in a microscope. A small speech balloon appears over the slide with the text, “The eye! The eye!”
Superman flying about a lighthouse, caught like a moth.
A waitress in a trashed up restaurant speaking to a police officer: “Well, first, this funny old sailor guy whips out a can of spinach, then this crazy music starts playing, and…well, just look at this place!”
Popeye on the stand at a murder trial. The DA saying “Your pipe was found at the scene of the murder. What kind of monster are you?” “I YAM WHUT I YAM!”
You know, I think the OP’s question is to some extent meaningless, but I submit to you that no one that I’ve never heard can possibly be the greatest cartoonist ever. Maybe your personal favorite, but not the greatest ever.
Cartooning is fundamentally a populist form of art. Overall popularity must be considered when judging the “best”.
Female ape grooming a sheepish looking male; “Well, well, another blonde hair. More “research” with that Jane Goodall tramp ?”
“Years later, while touring the Serengeti, Pinocchio’s wish to become a real boy was unexpectedly granted.” In the middle of a pride of lions, of course . . .
Two cavemen and a pterodactyl on a perch; the first caveman is saying “Don’t worry, me teach him to sit on finger.” He also has two peglegs, two pegarms, and an eyepatch.
What he said! 
My favorite:
A fish is holding a baseball bet, his head peeking out of the water. Just out of reach on the shore is the ball. The caption: “Great Moments in Evolution”
I think my favorite is “Cattle Drive Quartet”…four grizzled cowboys sittin’ around a campfire playin’ some Mozart:
“Gus! What the HELL you doin’?! This is Eine Kleine Nachtmusik, third movement, menuetto allegretto, you brainless horned toad!”
Yes, yes. I enjoy the scientists and survival themes.
“Fair is fair, Larry…” or “Notice Timmy, how the birds are so very territorial…”
Or…two geezers sitting on the park bench, feeding the pigeons. Two vultures are lurking on a nearby tree limb, however. “Now take them big birds Barney; never eat a thing, just sit and stare…”.