There are two types of people: people who can read and people who can look at pictures. Actually, there are three types of people because some people can both read and look at pictures. Graphic novels are good for the first two types, but if you can read AND see, then they're FANTASTIC for you! (Actually, I just realized that if you can't see, you can't read; unless, of course, you're reading brail. I am thinking about this too much now.)
Many of the titles on this list aren't actually one-shot graphic novels. Sometimes I list an entire series (usually a series that's concluded, but twice I rank an ongoing series). Basically, whatever's the best of the best. (I made the list so I make the rules!) Enjoy!
30.) EX MACHINA, Volumes 1-10 (complete series), by Brian K. Vaughan
29.) SIN CITY, by Frank Miller
28.) WANTED, by Mark Millar
27.) ULTRA: SEVEN DAYS, by The Luna Brothers
26.) POWERS: WHO KILLED RETRO GIRL?, by Brian Michael Bendis
25.) FROM HELL, by Alan Moore
24.) THE NOBODY, by Jeff Lemire
23.) THE ULTIMATES, Volumes 1 & 2, by Mark Millar
22.) WATCHMEN, by Alan Moore
21.) HELLBOY: SEED OF DESTRUCTION, by Mike Mignola
30.) EX MACHINA, Volumes 1-10 (complete series), by Brian K. Vaughan
29.) SIN CITY, by Frank Miller
28.) WANTED, by Mark Millar
27.) ULTRA: SEVEN DAYS, by The Luna Brothers
26.) POWERS: WHO KILLED RETRO GIRL?, by Brian Michael Bendis
25.) FROM HELL, by Alan Moore
24.) THE NOBODY, by Jeff Lemire
23.) THE ULTIMATES, Volumes 1 & 2, by Mark Millar
22.) WATCHMEN, by Alan Moore
21.) HELLBOY: SEED OF DESTRUCTION, by Mike Mignola
20.) PRIDE OF BAGHDAD, by Brian K. Vaughan
The story of a pride of lions who escape from the Baghdad Zoo during the bombing of Baghdad.
19.) MARVEL 1602, by Neil Gaiman
The title is pretty accurate; it's Marvel heroes and villains in the year (you guessed it) 1602. Beautifully illustrated by Andy Kubert. You don't need a detailed understanding of the Marvel Universe to enjoy 1602.