After giving my personal (and experience based) opinion, my best suggested resource for core lists of adult graphic novels for libraries was Stephen Weiner's wonderful The 101 Best Graphic Novels, but the most recent edition is from 2006.
A few days ago, after also being inundated with questions about the titles all public libraries should have, the folks over at The Graphic Novel Reporter posted this list of the core adult focused graphic novels and graphic novel reference sources. From their list:
We're unveiling our core list of graphic novels, and we've divided the list to accommodate differing levels of space and commitment. The most important thing to note: This is not a best-of list or a list denoting the most worthy accomplishments in graphic novels (although everything on each list is an accomplishment in some way, shape, or form). Instead, it's a list to help retailers and all those who are seeking to start a graphic collection to get a sense of the essential books to acquire. This is where to begin for those who want to make a small step (a core list of 10 books), a medium step (an additional 25 books), or a larger one (100 more).
The Graphic Novel Reporter was already a great resource, but now, they are making themselves indispensable. Click here to see the entire list and click here to see my very popular Graphic Novels for Grownups annotatetd list on the Browser's Corner blog.
We begin here with adult-focused graphic novel titles, including reference books about the format. In the weeks to come, we will add titles for children and teens, as well as manga broken out by adults, children, and teens.