Superman vol.1 #30 - Cover date September-October 1944
Lois stars in her third solo installment, and features prominently in the opening story to this issue (which involves Superman and Clark Kent switching roles, or at least pretending to, for Jimmy’s and Lois’ benefits), followed by Superman helping set a rightful heir to a foreign throne back in charge of the country which was stolen from his ancestor.
More importantly than any of these, though, is the comic book debut of Mr.Mxyztplk, and - unlike his newspaper appearance - this story focuses entirely on his peculiar brand of mischief.
Fans of the animated Superman series from the 1990s will recognize some of the elements of the story, from Mxyztplk’s bulb-headed, purple-suited appearance to his faking death after a truck collision and, of course, loudly awakening the forgetful “McGurk” (actually the famous Rodin statue The Thinker) at a Metropolis museum.
The incredibly popular imp causes a number of ruckuses around the city, engages Superman in a super-breath contest - not something that happens every day - and eventually laughs himself right back to the magical dimension of Zrrrf - for a short while, anyway, as he quickly proves to be one of Superman’s most popular foes.