One of my early favourites is the story "Do Not Panic!" which first appeared in Strange Tales #95 (1962), although I first encountered it as a reprint in Curse of the Weird #1 (1994). Stan Lee was scripter and signed his name, as he often did on Ditko tales.
From a worm's eye perspective we see people panicking as an otherworldly craft descends from the skies. From there the perspective moves to people indoors listening intently to radio broadcasts of the craft's descent; people lock their doors. A squad of tanks arrive to encircle the craft at its landing sight. It's very much in the spirit of The Day the Earth Stood Still.
A lone astronaut emerges from the craft, clad from head to toe in a spacesuit. The wary soldiers demand he drop his gun belt; the astronaut politely complies. Offering friendship, the astronaut removes his helmet to reveal he is a normal-looking human man. The soldiers are Martians and the setting has, from the start, been upon Mars rather than Earth.
"Do Not Panic!" is only four pages and it's the right length. The story has a very simple premise and the tale gets in and out right on cue. Ditko was very careful to avoid depicting the Martians' faces until the last panel, but he did so in ways which would not tip the readers off to the twist - the lack of faces simply adds to the sense of paranoia gripping the populace. This tale is representative of many of those which Ditko produced for Atlas (nearly all of them scripted by Lee). These tales were almost all based around a very simple twist ending, but sometimes that twist could be unclear or require too much explanation at the climax. Here, the twist is told entirely through visuals, not text and that makes this a particularly special thing in comic books, a medium where all-too-often emphasis is placed on text instead of image.
Do enjoy your ninetieth year, Mr. Ditko.