Steve Ditko will be remembered most for co-creating Spider-Man and Dr. Strange with Stan Lee (and may deserve more of the credit than Lee, depending on who you ask). He drew some of the most loved Spider-Man stories but some of the absolute best Dr. Strange stories. That's rare for creators—most later artists try to one-up the creator and many succeed, but only with Dr. Strange do all later creators merely present inferior imitations of Ditko. This is perhaps the biggest difference between Ditko's legacy on Spider-Man and Dr. Strange. Spider-Man became the face of Marvel, but Dr. Strange has remained a relatively obscure specialty title about which you might say, "Spider-Man is great standard superhero stuff, but if you want a real advanced, mind-blowing experience, you have to check out Ditko's Dr. Strange." Since Ditko perfected it right out of the gate, I'm tempted to say Ditko ruined Dr. Strange by being so good, but I don't want to belittle the other great artists who worked on the title. Gene Colan and Frank Bruner are iconic; Paul Smith, Michael Golden, Kevin Nowlan, P. Craig Russell, and Chris Warner are all magnificent; Chris Bachalo and Peter Gross are two of my personal favorites—the list goes on and on. I love most of the artists who have worked on the title, but I think even they would admit they're merely shadows of Ditko.
Just as Ditko gave Spider-Man one of his greatest moments of all time, lifting up the machinery in The Amazing Spider-Man in 1966, around the same time Ditko gave Dr. Strange one of his greatest moments: meeting Eternity for the first time. It comes near the end of the greatest Dr. Strange story: The Eternity Saga. The story started in Strange Tales 130 and lasted about twenty issues/two years (a rather long story in those days) in which Doc’s two greatest enemies, Dormammu and Baron Mordo, team up and chase him all over existence in their mania to destroy Doc's inferior power and Doc's stubborn refusal to surrender to obsolescence. Meanwhile, Doc’s normal deus ex machina, the Ancient One, is nearly catatonic and mumbling about Eternity. It sets up several patterns for Dr. Strange stories to follow. Part of the appeal of Dr. Strange is his Sherlock Holmes-like ability to solve any problem with his vast intellect and a vast power base (dozens of spells and hundreds of magical objects packed into that crazy house) that sometimes mitigates his vulnerability. This may make him seem cold and unrelatable to casual fans, but Doc's best stories take him to the limit of this supposedly god-like power level, and nobody ever pushed him quite as far as the Mordo/Dormammu team up. The high point of this journey is seeing the face of Eternity for the first time. This makes a perfect goal for this story (showing Doc doesn't actually know everything yet) but undermining later stories (because maybe now he does know everything), though the best Dr. Strange stories still attempt to follow this same pattern.
Beyond these two iconic characters, Ditko established a cosmic pantheon in the pages of Dr. Strange that other later writers developed, giving Marvel one of the richest mythologies of any fictional universe. When Dr. Strange finally figures out that the Ancient One's muttering about eternity is actually about Eternity with a capital "E," an anthropoid entity embodying all of existence, this moment essentially inaugurates, with Ditko's signature psychedelic style, Marvel's unique version of the gods (and maybe also God with a capital "G"). The pantheon of Marvel’s multitudinous cosmic entities reach full flower in the 70s in stories by Jim Starlin and others, but the seed for this cosmic pantheon was Ditko's imagination (in a backup story in Strange Tales of all places). Marvel had plenty of sci-fi before this point, but there's a difference between traveling through outer space, meeting aliens, traveling through a conceptual plane and finding out why outer space exists in the first place. Fair credit should go to Jack Kirby, one of the few artists who could challenge Ditko's place on the pinnacle, as Kirby created Galactus and other similar characters in Fantastic Four and other titles, but one could easily interpret Galactus as a traditional invading alien on a larger scale; it's hard to mistake Eternity as anything other than the embodiment of all existence. An early conception of the interaction of godlike entities was that Eternity, Death, and Galactus as the core trinity of all of everything. And Doc would eventually become one with both Death and Eternity under Steve Englehart (The official poem of Doc in the Ditko Era should be “Sha-Clack-Clack” by Saul Williams: “Before Death is Eternity, after Death is Eternity and I’ll be riding on the wings of Eternity like Hyaa! Hyaa! Sha-Clack-Clack!”)—but then a short time after the Eternity Saga in Strange Tales, Doc would introduce readers to the Living Tribunal, created by Marie Severin, not Ditko, but clearly under the influence of Ditko (as much as I love the story that introduces the Living Tribunal, it's essentially a riff on the Eternity Saga, with Umar replacing Dormammu, and Marie Severin doing her best to one-up Ditko's Eternity design with all the weird design touches in the Living Tribunal, and she arguably comes closer than anybody). Besides an essential example of Ditko's influence on later writers, the Living Tribunal shows how Ditko expands the cosmic questions comics are capable of— from "What do aliens look like?" to "What does God look like?"