You can't discount the contribution of Steranko.
Oh yeah, lots of great artists have illustrated the comic: Frank Brunner, Gene Colan, etc.
But I believe it was Ditko who originally established the distinctive psychedelic look of the series . . .
I'm not so sure -- that's why I mentioned Steranko. His name and "psychedelic comic" sort of run together. Of course, all these guys in the Marvel bullpen (back when there was such a physical place) were influencing each other...
I haven't looked it up, but my impression is that Steranko came along later, some years after Ditko created the look of Dr. Strange's universe . . . .
ADDITION: Okay, I looked it up. Dikto did DR. STRANGE from 1963 to 1966, before Steranko was hired by Marvel.
Steranko took over NICK FURY, replacing Jack Kirby, around 1966. That was where he introduced all the psychedelic stuff. He didn't have anything to do with the visuals in DR. STRANGE. That was all Ditko.