I'd say regarding Alan Moore's work, it's more of a four-way toss-up between his masterworks on SWAMP THING, MIRACLEMAN, WATCHMEN, and my personal favorite of the four, V FOR VENDETTA.
Also high on my list of favorites of Moore is his "last Superman story" tribute to the Swan/Anderson-era Superman in SUPERMAN 423/ ACTION COMICS 583, reprinted in the SUPERMAN: WHAT EVER HAPPENED TO THE MAN OF TOMORROW trade.
Frank Miller's DARK KNIGHT RETURNS and BATMAN: YEAR ONE rank very high up there also.
And his DAREDEVIL: BORN AGAIN (from DD issues 226-233).
And although seldom mentioned by anyone, the first five volumes of Dave Sim's CEREBUS are, for my money, the equal of the best of Alan Moore's work. Very fun and intelligent reading.
And Don McGregor/Craig Russell's AMAZING ADVENTURES/ Killraven series (issues 18-39).
But I think for many of us, that "one point" in comics could just as easily be the Lee/Kirby Marvel titles (particularly THOR and FANTASTIC FOUR).
Or the Schwartz-edited Silver Age DC titles (I especially enjoyed Adam Strange/MYSTERY IN SPACE, FLASH, DETECTIVE COMICS and JLA).
For me personally,
it's all About Jack Kirby and Neal Adams, and all other greatness descends from their groundbreaking work.
And as refined as the newer titles are (at least up through about 1990, when I mostly stopped caring) I gain more enjoyment from reading the earlier works again:
Neal Adams Deadman (STRANGE ADVENTURES 205-216)
Neal Adams BATMAN (issues 219, 232, 234, 237, 242-245, 251, 255)
and DETECTIVE COMICS (issues 395, 397, 400, 402, 404, 407, 408, 410, and loosely credited in 439)
Jack Kirby's fourth world books (1970-1972) :
FOREVER PEOPLE 1-11
NEW GODS 1-11
MISTER MIRACLE 1-9
JIMMY OLSEN 133-148
And even more so for me, Kirby's DEMON, KAMANDI and O.M.A.C. series.
KAMANDI especially is exactly the type of fun, heroism and adventure I love comic books for, as an experience unique from any other medium. The mind-blowing images and double-page spreads, likeable characters, pure fun and adventure.
I think there's general agreement on many other masterworks in comics, such as:
Wein/Wrightson SWAMP THING 1-10
O'Neil/Kaluta THE SHADOW
Goodwin/Simonson MANHUNTER (DETECTIVE 437-443)
Kubert TARZAN 207-235
Thomas/Smith CONAN 1-24 (and virtually all of the Buscema work I love too)
Starlin's CAPTAIN MARVEL 25-34
Starlin's WARLOCK (reprinted several times since they first appeared in 1974-1977)
Starlin's DREADSTAR / THE PRICE / METAMORPHOSIS ODYSSEY
Moench/Bolton's KULL: "Demon in a Silvered Glass" (BIZARRE ADVENTURES 26)
Moench/Gulacy's MASTER OF KUNG FU 18-50 (with a lot of fill-in issues)
Moench/Day's MASTER OF KUNG FU 102-120
Goodwin's EPIC ILLUSTRATED magazine
Moench/Sienkiewicz's MOON KNIGHT 1-30
Bruce Jones/Brent Anderson's KA-ZAR 1-27
Bruce Jones-edited Pacific Comics titles, TWISTED TALES, ALIEN WORLDS, SOMERSET HOLMES, BERNI WRIGHTSON:MASTER OF THE MACABRE, SILVERHEELS, etc.
Miller's DAREDEVIL 158-191, 225-233
Claremont/Byrne/Austin's X-MEN 108-143
Michelinie/Romita Jr/Layton IRON MAN 116-156
Stern/Romita Jr. AMAZING SPIDERMAN 224-251
Stern/Rogers, Golden, Paul Smith, etc's DOCTOR STRANGE 46, 48-73
Levitz/Giffen LEGION OF SUPERHEROES 285-306
Walt Simonson's THOR 337-380
Giffen/Dematteis/Maguire/Hughes JUSTICE LEAGUE/JLI/JLE, and all its annuals, specials, quarterlies, and what-not.
Perez's WONDER WOMAN
Tim Truman's HAWKWORLD miniseries.
All of these are very well written definitive runs on these series and characters.
It's hard to choose just one, from all these great stories, and more.
If I had to choose one, I'd probably choose a Kirby or Adams story.
But I certainly wouldn't object to an Alan Moore book being chosen either. WATCHMEN's a good choice.
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"This Man, This Wonder Boy..."