Asterios Polyp by David Mazzuchelli
Asterios was published fairly recently and is one of those widely accepted, critically acclaimed (even reviewed by The New York Times Book Review) novels that would work really well for groups that already have an interest in discussing literary fiction. It is rife with metaphor and the artwork is a unique and essential part of the story.
Palestine by Joe Sacco
For those reading groups that enjoy discussing nonfiction, most of Sacco's work will be very appealing. Sacco is a comics journalist - he has spent time in Israel/Palestine and this particular work is a reflection on his experiences and the plight of the Palestinian people. Other works by him to try would be Safe Area Gonazde (about the Bosnian War) and Footnotes in Gaza.
Persepolis by Marjane Satrapi
I'm guessing everyone has heard of this one by now, but I want to mention it because it is ideally suited for reading groups and just a fantastic book. There are two volumes, but there is an edition that collects both into one volume. It is about Satrapi herself growing up in pre- and post-revolution Iran whose family deals with the fear-instilling, fundamentalism of the Iranian government. The second volume focuses more on Satrapi's experiences outside of Iran as a teenager. Satrapi is a wonderfully interesting, funny, deep, and emotional character that will appeal to most.
Transmetropolitan by Warren Ellis
Ellis is a powerhouse in the industry and this science fiction series is perhaps for what he is known best. Spider Jerusalem is a journalist and all around interesting character living in a dystopian future world. This is a 10-volume series, but don't let that stop you - the volumes are short and choosing to read just the first volume is usually the best way to go. You'll find that most people will come having read multiple volumes (each volume is a pretty quick read) and the trick will just be avoiding spoilers.
Watchmen by Alan Moore
This is a classic and a great introduction to graphic novels. It is a bit of a spoof on the typical superhero genre: the characters reside in a world where superheroes have played a significant role in history and have now been outlawed. The movie is good, but the book is far better because it reveals how multi-layered this novel is. It truly does what only comics can do!