The Saga of the Swamp Thing by Alan Moore, et al.
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So this was the comic project that Alan Moore made his name on before Watchmen and its a doozy. Forget anything you may think you know about Swamp Thing, this is a psychedelic, wrenching, supernatural love and horror story that happens to be about a swamp monster.
One of the most interesting things about the Saga is that Moore started writing Swamp Thing midstream on an established series, rather than starting fresh (like he did with Watchmen) or rebooting an entirely moribund franchise (like Gaiman did with Sandman). So there are a fair number of "loose ends" left over from the previous continuity, and the rest of the DC comics universe continually intrudes on the narrative. These incursions are sometimes sensible and organic, but other times are tonally peculiar to say the least. What's neat about that, though, is that Moore incorporates all those disparate elements and transfigures them in ways that enhance the story. Even Batman is forced to take the Swamp Thing on his own terms and it's gloriously weird.
And then there are the moments where Moore pushes the narrative entirely outside the essential banality of superhero comics into serious social issues or truly phantasmagorical material. In the Saga you'll find observations on the psychology of domestic abuse chock-a-block with hallucinogenic sweet potatoes; rhyming arch daemons a few issues away from elder abuse. And then there's the overwhelming and pervasive body horror: plants growing into or out of people, people made into plant people, computer/plant hybrid aliens extracting elements from plant people, normal meat-based aliens extracting elements from normal people, people entirely consumed from within by alien creatures, plant people forcibly made into larger plant people. If you can think of some horrifying way for meat and plants to interact with other meat or plants, it's probably in here somewhere.
It's probably not a good "first graphic novel" for someone who has never picked up a comic before, because even bracketing out the horror, its much less self-contained than Moore's later work (and one needs a little background to appreciate all the crossover characters and how weird the uses Moore has made of them). That said, it's a really, spectacularly excellent graphic novel arc if you like horror comics even a little, and is (in my opinion) a must read for fans of the darker side of the DC comics universe.