There was one mention of The Nameless City in the thread, but, really, ANYTHING by Faith Erin Hicks would be a worthwhile read. Superhero Girl, Friends with Boys, and Nothing Can Possibly Go Wrong are her most recent ones (after the Nameless City series, of course [side note: it just got signed to be adapted as an animated show!]).
My personal all-time favorite graphic novelist is Craig Thompson. However, his work is mostly geared towards an adult audience (Blankets and Habibi are definitely NOT for kids). Space Dumplins is would be excellent for upper-elementary age, though.
I've recently been going through the Scott McCloud how-to books in an attempt to up my game as I am currently working on the script for a GN (long ways to go yet). Recently, my biggest comic consumption has been through webcomics (Prague Race, Sulfur and Salt, Vattu, Mias & Elle, The Meek, and Mare Internum are some of my favorites).