Amulet: The Stonekeeper; The Stonekeeper's Curse; The Cloud Searchers; The Last Council; Prince of the Elves by Kazu Kubuishi
This is probably the most popular graphic novel series in our library (with the possible exception of Jeff Smith's Bone). And with good reason. The illustrations are very detailed while maintaining a cartoony style. The story is told in words and images in equal measure. Add in complex world building and characters who change and grow, and this is pretty much the ideal graphic novel series for middle grade readers. Besides, who wouldn't love a floppy eared bunny sidekick named Miskit?