see, i tell no lies. Kingdom Come is easily the best comic I've ever read. Ok, not easily. Watchmen and Maus both come really close if not surpass it. but yeah, one of those three, no hesitation.
If your library has them, might I recommend some other graphic novels? (in addiction tot he above)
Thrillkiller (DC, Elseworlds) This is essentially what Tim Burton would have done with Batman if he had made the first Batman movie thirty years earlier. Except BATGIRL is the one throwing the team together. Interesting stuff.
Cerebus (Dave Sim). Any of them, but they're best if read in order. Sim takes an Aardvark furry spoof of Conan and plays with it for a couple years, then gets genius and runs with that baggage with Cerebus in the real world, and some very interesting things happen. It's not typical comic book art for the most part, but he knows how to use the images and he has some of the most interesting ideas for his foundation.
Kabuki: Circle of Blood (David Mack). A fusion of manga and American comic styles, it has partially a cyberpunk feel crossed with superspies. It's futuristic, and dark. Kabuki is an agent for a secretive government agency. An assassin witha personal vendetta. He keeps bringing up great ideas for the seting. But my favorite reason to read any Kabuki story is to look at how he fuses his art and text completely. this is where comics were meant to go.
Sandman (Neil Gaiman) (I think Tage has mentioned owning most of these) You know neil Gaiman from his prose. This is what really got him started. He manages to break away from traditional art AND storytelling, absorb some mythology, and still take off from a starting point of superheroes to make a completely original character and story.
Pretty much anything by Will Eisner, but particularly Last Day in Vietnam. see comments on will Eisner below. This one is a culturally and historically charged look at a soldier's last day on his tour of duty.
for curiosity's sake, Batman: Hong Kong. Not always clear that the multiple art styles merge, but both styles are conducive to the story and are fascinating to look at. The plot? Batman goes to Hong Kong to trap a snuff film engineer, andcan't get anywhere without the help of a newbie hero that he inspired.
I, Joker (DC, Elseworlds) in the future, the hero of Batman is worshipped, and "criminals" surgically altered to look like the 20/21st century Batman's rogues gallery are released annually for the inheritor of the legacy to hunt down-- only to preserve the cult status. Classic story of one man's insanity bent to goodness, bent back by others to preserve their power.
JLA: The Nail (DC, Elseworlds) This is more of a what if? story than anything else. What if isolationit Amish pacifist had found baby Kal-el in the field instead of Ma and Pa Kent? that's right, the JLA falls abart 30 years later to the machinations of Lex Luthor.
Metropolis (DC, Elseworlds) Cross the old silent movie with the idea of superman, and you get this. I totally didn't understand it until after I watched the film, but it's a very interesting setting. Not as great as some other stories, but intersting.
I can annotate to say why I like them if you would like)