I wouldn't (being a geek and former Comicshop owner) blow off the graphic novels so lightly. The films of V for Vendetta and Watchmen bare no relation to nor are anywhere near the same league as the original books. Both of these are incredibly deep and layered tales. V for Vendetta especially (a dystopian tale about a terrorist/freedom fighter in a 1984esq world).
Sandman is written by Gaiman and covers a whole range of different styles of writing, art and storytelling. I think personally it is a far superior work to his later novels.
Batman Year one and Dark Knight Returns are superhero comics but are considered to be the last word in them. Year One follows the parallel tales of Jim Gordon coming to Gotham and Bruce Wayne returning and it is brutal, gritty and very down to Earth. DKR is written as the last Batman tale and features an aged Bruce Wayne returning to the mantle. It is a parody of American culture and the American dream.
Watchmen and any of Alan Moore's work from the 80s and early 90s are well worth a look. Watchmen transcends the superhero genre and is a metatextual piece that plays with time, colour, style and even genre.
From Hell is a telephone book sized pHD paper on Jack the Ripper in graphic form (and again bares utterly no relation to the film based on it).
Strangers in Paradise is just wonderful. That's all.
Most are available from the library if you don't want to buy.
Slaughterhouse V is pretty much my favourite book of all time; So it goes.
Iain M Banks and his nonsci-fi alter ego Iain Banks are well worth a look he is dark in either guise but often in very different ways.