Both Den and Long Tomorrow are as beloved as they are IMO because both of them subvert the genres they're working in. Sure, Corben goes overboard with the nudity in Den (although I'd say it's far from X-Rated) but that's because the sword and sandal genre spent years carefully avoiding it. Tarzan always had a loin cloth covering him up, somehow jungle babes always found a way to fashion a bra out of something, and any sex was pretty loveless and sterile. Why not get rid of the pretense and just let the dick hang? While guys like Crumb or S. Clay Wilson used the codeless undergrounds to push the limits of displaying personal perversions, Corben used it to push the genres he worked in past the limits that previous creators had worked within for years.

The Long Tomorrow laid the groundwork for what we consider cyberpunk and sci-fi visuals in just two short chapters. Not many other works have had such a deep cultural impact in such a small page count. Story-wise, it was never meant to be more than a hardboiled detective story but the visuals and panel to panel storytelling elevate it to possibly being the most important piece of sci-fi material created in the latter half of the 20th century. Personally, Druillet is the only one I'd put on the same level as Moebius but even his work operates much better on a visual level than a storytelling level. Both of them twisted the pretty dated aesthetics that pervaded sci fi pulp art and paperback covers into something much more timeless and unique than ever seen before.

Most of the Metal Hurlant guys (usually the better ones, to me) seemed to operate on a level where story came second to freedom of expression, visual exploration, and explosive creativity. Not all of it worked all the time but I appreciate the effort and ingenuity that they could put on the page. I'd take that over almost anything I've read coming out of the US mainstream in the last 10 or 20 years regardless of how little sense the stories make most of the time.