I think Frank Miller's
BATMAN: THE DARK KNIGHT RETURNS, while not technically an Elseworlds or a series having the Elseworlds logo on the cover, was in any case the first Elseworlds, the book that created the squarebound high-quality format, and the template of presenting DC heroes in alternate realities with aspects changed, in a limited series or one-shot format.
I'm not entirely clear whether BATMAN: YEAR ONE was an Elseworlds type story, or a full-fledged reboot. I don't recall it being rigidly held continuity in subsequent issues, but maybe it was.
Anyway, these two series by Frank Miller, along with Alan Moore's WATCHMEN and V FOR VENDETTA, I think of as the
Pink Floyd: Dark Side Of The Moon of comics. Books that remain top sellers over 30 years later, and are constantly back in print.
DARK KNIGHT RETURNS is certainly a good book to argue is the starting point of the Elseworlds line.
