Okay, this is highly subjective of course but Alan Moore’s Miracleman and Frank Miller and Bill Sienkiewicz ‘s Elektra: Assassin might be the best comic books ever made (or, at the very least, my favorites). The plots are intricate and evocative, the scripting and dialogue is unique, and the art in Elektra Assassin and Miracleman (specifically the issues drawn by John Totleben) is innovative and often breath-taking, moving way beyond comic book art into realms of finer art and illustration.
So of course there are runners up which others may find more important to them: The Watchmen, V for Vendetta, Batman: The Dark Knight Returns, Akira (not that the writing in Akira was great by any means, but for the time the novely of the Japanese elements and setting, the great art, and some of the ideas, in a pre-anime-popularity era in the US, was pretty noteworthy). Note that most of these comics are also by Frank Miller and Alan Moore, who in the 1980s and transformed the notion of what comics could be and created truly graphic “novels” instead of just “comic books”. Some of the writing and ideas were to standard comic book fare what Kurt Vonnegut and Don DeLillo are to more typical fiction, what David Cronenberg is to more typical movie directors, what Voivod’s Killing Technology, Dimension Hatross, and Nothingface were to typical metal in the 1980s, and what Sonic Youth’s Daydream Nation was to typical punk and guitar pop when it came out in 1988. The 80s, what a heady time for art and culture the 80s were!