What: The final sentence spoken by Ozymandias in this panel from Watchmen #11. Sorry, I can't tell you what it is.
Nope, sorry, still can't tell you. I don't want to ruin the ending. But if you search the Internet for Ozymandias and Watchmen, there's a really good chance you'll find it. But don't say I didn't warn you.
Why: The period of 1985-1988 was the true Golden Age of comics, especially for DC, which spiraled out Crisis on Infinite Earths, Legends, and The Dark Knight Returns in a very short time. Each of these was a grittier view of the superhero universe, one where the very nature of superheroism made little sense against the backdrop of a world with real problems. At the top of the mountain was Alan Moore and Dave Gibbons' Watchmen, a 12-issue miniseries set in an alternate 1985. The Soviet Union and the United States are on the brink of war in Europe, and all superheroes have been declared illegal. A hyper-brilliant hero named Ozymandias has an intricate plan to set it all aright, but his fellow masks discover his plan. Ozymandias then says the line to end all lines, and your jaw hits the floor.
Impact: This line ripped the mask away from comic book story writing. It told you, "Every comic you have read in your entire life has held your hand, coddled you, treated you like a child. No longer." Comic writers now had the freedom to write whatever they wanted. Neil Gaiman, Garth Ennis, and Art Spiegelman showed up. Comics became literature, and soon would become one of the most powerful cultural forces in America. (Sadly, it came at a price: Traditional comic storytelling faltered. Superman got killed, Batman got snapped in half, Spider-Man got cloned. And I quit reading comics.)
Personal Connection: I have eight boxes of comics in my basement. The trade paperback of Watchmen is the only one that sits on my bookshelf. I've lent it to more people than I can count. When it comes back, the borrower always tells me about what happened when they read the line. How they had to flip backward and read it again. So that's why you know the line if you've read it, and I'm not telling you if you haven't. Come by and I'll lend it to you so you can see for yourself.
Other Contenders: "Stupid, stupid rat creatures!!" (Fone Bone in Jeff Smith's Bone #31); "Face it, Tiger… you just hit the jackpot!" (Mary Jane Watson in Stan Lee and John Romita's Amazing Spider-Man #42); "When the mob and the press and the whole world tell you to move, your job is to plant yourself like a tree beside the river of truth, and tell the whole world, 'No, you move.'" (Captain America in Amazing Spider-Man #537); "I want you to remember, Clark… in all the years to come… in your most private moments… I want you to remember my hand at your throat… I want you to remember the one man who beat you…" (Batman in Frank Miller's The Dark Knight Returns #4).