Longtime contributor to The Jack Kirby Collector Sean Kleefeld has launched a Kirby character design meme, and that meddling Canadian plok has not only joined in with his choice but roped me into participating as well.
I couldn't pick just one single character as being Jack Kirby's single best visual design: there are so many of them that have become iconic and stood the test of time, as demonstrated by the fact that every subsequent artist to draw the character has followed Kirby's original design faithfully. Galactus is one of those characters who will always look like the Kirby original no matter who draws him. So is the cult favorite villain Modok. In design terms these guys are like the original Volkswagen Beetle or the first iMac, with its white-and-Bondi-blue case -- they came seemingly out of nowhere and instantly defined an idea space so completely that other designers couldn't improve on them or even evolve them much.
I could very easily pick Galactus as my choice here, but I have another "dark horse" candidate in mind: another character who has never been improved or altered, but worked perfectly on his first appearance and looks as striking today as he did back then:
Even in repose Black Bolt, King of the Inhumans, is dynamic. The ribbed glider wings and flaring gloves (the latter a bit underplayed on this cover) mean there are always lines radiating from his figure, and as a result he dominates any panel in which he appears, the way his character dominates his subjects. By contrast it seems as if two of his Inhuman subjects, Karnak and Gorgon, are almost always drawn in a slight crouch, lower than their monarch.
The electron-gathering antenna on his forehead is primarily there to look cool -- though Lee and Kirby gave it some rationale so that we could see it crackle with Kirby dots -- and the ziggurat trim on his uniform is solely decorative. Basically, he's the best-known Art Deco character ever in comics.
It's interesting that although Black Bolt is not a superhero per se, Kirby designed him with superhero tights and mask. A number of the Inhumans -- among the leads, Medusa and Karnak as well as Black Bolt -- always wear tights and masks. The Inhumans were Kirby's first use of an idea he'd return to with the New Gods and the Eternals: characters who were not crime fighters with secret identities, but who borrowed the trappings and design elements of superhero comics simply for their visual appeal as sheer ornamentation. I don't think the continuing primacy of superheroes in comics can solely be laid at stunted development and innate conservatism on the part of creators and readers. Superheroes (or at least characters with a lot of the features of superheroes) are one of the ideas that just work really well in comics and use the distinctive traits of the medium. But with the Inhumans -- with Black Bolt especially -- Kirby started exploring how the visual conventions of the superhero could be applied to other sorts of characters who could be used to tell other sorts of stories.
When other creators have taken Kirby characters like these and just used them as slightly more outre and unusual superheroes, they've totally missed the point. We didn't need Orion and Big Barda and Mister Miracle in the JLA or Sersi in the Avengers, putting the smack down on super-villains. These characters represent a new genre. Yes, I know Medusa got her start as a member of the Frightful Four...but even in the original Lee-Kirby issues of FF she shucked that off quickly for a more interesting role.
And a side note on Black Bolt: it's well known among Kirby fans that Jack had a lot of trouble drawing Spider-Man. He managed it for the cover of Spidey's first appearance...but once Steve Ditko defined how the costume and the character's body language worked, Kirby could never do it that well again. This always struck me as odd considering the way Joe Simon and Kirby owned the "slim athletic superhero in tights" all the way back in the Forties with characters like Manhunter. Look at how well Kirby handled the yellow-and-purple-tights version of Sandman, swinging across the city skyline with his wirepoon gun, and explain why he could never do Spidey well. I suspect that when another artist had defined a character in Kirby's mind, he became inhibited...and Kirby was the antithesis of inhibition.
By the same token, Kirby never drew a persuasive Batman...but his Black Bolt looks more like Batman than any drawing of Batman Kirby ever did. If he'd drawn Black Bolt instead and merely altered the costume afterward, we might have seen a very different result.