We're in the top five now!
5. Alan Moore's Swamp Thing - 942 points (30 first place votes)
Saga of the Swamp Thing/Swamp Thing #20-58, 60-61, 63-64, Annual 2
Something that I think often gets lost when discussing Alan Moore's tremendous run on Swamp Thing is how the run started. Everyone remembers Moore's SECOND issue, "Anatomy Lesson," but Moore actually started on the book one issue earlier, tying up the loose plotlines of previous writer Marty Pasko. It's quite interesting to read the care and attention Moore puts into Pasko's storylines, while still managing to wrap it all up in one issue in a much different style than Pasko.
And then, of course, with the Pasko storylines finished, Moore dropped the big one - "Anatomy Lesson."
There have been a number of other significant retcons with titles before (Anyone remember Steve Gerber's strange retcons during his Captain America run?), but they all paled in comparison to what Alan Moore did with "Anatomy Lesson," which revealed that the entire origin of Swamp Thing was false - Alec Holland was not transformed into Swamp Thing during a chemical explosion - instead, the chemicals animated a group of vegetation into THINKING it was Alec Holland.
Later, Moore would also explain the various inconsistencies of Swamp Thing's origin by saying that there were many different Swamp Things who all had the same basic origin. Clever meta-fiction work by Moore.
Moore was ably assisted by the art team that was there when he joined the book, penciler Stephen R. Bissette and inker John Totleben - together, Bissette and Totleben delivered a stunningly rich art style, that was perfect for the moody stories Moore told.
Throughout his run, Moore would tell deep character-based stories, most notably the relationship between Swamp Thing and Abigal Arcane (being one of the first comic to show two characters having sex, although in a rather odd manner, seeing as how he is a plant). Also notable in Moore's work was when he would touch on the DC Universe, and give us drastically different takes on various famous superheroes. Moore's early work with the Justice League in an issue of Swamp Thing informs pretty much every modern writer of the Justice League.
During his run, Moore also introduced John Constantine, who would be Swamp Thing's guide on a number of stories (more accurately, he would con Swamp Thing into getting involved in stuff).
Towards the end of his run, Rick Veitch became the artist on the book, and while he had a much different style than Bissette and Totleben, it was still excellent artwork, and it was Veitch who would end up following Moore on the title as writer (and doing a fine job, himself).
Without Moore's Swamp Thing, we likely wouldn't have seen Vertigo and all the comics that spun out of Vertigo, or if we did see them, it would have taken a long time to get there, so its influence is massive.