I agree with all the recommendations of Alan Moore's
Swamp Thing. One of the best comics I've ever read.
And for some Superman recs - I noticed you mentioned being a fan of
Kingdom Come, so I'd also recommend
Superman: Birthright, which I can't believe I haven't seen mentioned here yet.
It's a modern Superman origin story done by the guy who wrote
Kingdom Come, Mark Waid. My personal favorite of all the Superman origin tales.
And one that I
have seen mentioned a few times in here, with good reason:
Superman: Red Son. A brilliant alt-world tale and the only thing I've ever truly loved from Mark Millar. By transplanting Supes to Soviet Russia, Millar manages to say more about who the Man of Steel is at his very core than most in-continuity books do, imo.
And finally, my all-time favorite superhero comic:
All-Star Superman. This book is pure Love. Grant Morrison is some kind of wacky genius.