Sandman was assigned reading in a Pop Culture course I had taken in my last year of uni. Phenomenal. I haven't yet gotten around to reading anything else by Neil Gaiman (American Gods is next on my reading list), but it did pique my interest in graphic novels--the term that cuntish pedants use to describe works of sequential images & text which are not about superhumans prancing about in their underwear. Although Sandman takes place within the DC comics universe, to call Sandman a comic book is the sort of thing that inspires apoplectic rage among certain geeks. Not me, but I'm not shy about pedantry.
Preacher is remarkable. It subverts Wild West tropes and cowboy mythology much the same way Sandman subverts those of comic book superheroes. Lucifer, a spin-off of Sandman, follows the eponymous character's exploits after certain events in the main continuity. Transmetropolitan is for fans of cyberpunk and dystopian science fiction as well as anyone who enjoyed Fear and Loathing in Las Vegas or other works of the late great Gonzo journalist Hunter S. Thompson. I still haven't read Watchmen, although I found the movie quite tolerable. I suspect that would ruin the comic--ahem, graphic novel--for me.