Depending on if you want all ages or not... *'s indicate not for little kids... quickly, off the top of my head:

Any Marvel Masterworks or DC archives
Marvel Epics (especially those that don't overlap the Masterworks)
Fables*
Preacher*
Walking Dead*
Transmetropolitan*
Watchmen*
V for Vendetta*
Saga*
Bone
Cleopatra in Space
Usagi Saga volumes
Giants, Beware (and Dragons, Beware)
Maus
Persepolis
Barefoot Gen
March
Five Ghosts*
Lazarus*
Any recent Marvel and DC the readers want

All good suggestions so far, but I'd also suggest long runs of great strips. Maybe not technically "graphic novels," but if the librray does not have, say, a few collected editions of Prince Valiant, Gasoline Alley, Pogo, Calvin and Hobbes, Peanuts, Dick Tracy, and so on(not the entire runs, but representative volumes) this might be a way to squeeze them in.

Also:
Little Archie
Any Joe Kubert works: Yossel, Fax From Sarajevo, e.g.

I'd also suggest a few GNs about reading GN's to help people unfamiliar with the medium-McCloud's Understnading Comics and Eisner's Graphic Storytelling come to mind as absolute necessities for any GN collection.

I'd also suggest the collection have a diversity of era and creators. Make sure the collection doesn't degenerate into all capes or a collection of stuff by "dead (or even still living) white guys"

I know you said no super-hero stuff, but of the super-hero stuff they had I would make sure they had the Golden Age Batman and Superman Omnibus to make sure the origins of the super-hero genre were covered, as well as something with the early Marvel/Marvel Mystery comics.

Some of the collected EC Editions to show where comics went in the post-War era. Weird Science, Tales from the Crypt, Mad, one of the crime and war book collections so all genres are represented and sampled.

As Hal pointed out-some of the classic strip collections-Popeye, Pogo, Terry and the Pirates, Flash Gordon, Prince Valiant, Peanuts, etc.

A few key Silver Age collections as well to represent the era.

European Comics as well-Asterix, Tintin, Corto Maltese, among many others. Collections of key or influential manga or those of historical importance also, maybe Astro Boy, Lone Wolf and Cub, Barefoot Gen, Akira, some others.

Some sampling of undergrounds without going to crazy at first-maybe Golnick's History of the Universe, Crumb's Book of Genesis that at least touch on material people are familiar with to give them a taste of how the underground's approach things. Definitely need to get some Harvey Pekar in there as well before moving into the more out there or modern stuff.

Pivotal stuff that changed the perception of comics in the mass media-Maus, Fax from Sarajevo, Safe Area Gorazde, Persepolis, etc.

Stuff that takes new cultural looks at western canon-(perhaps the manga Shakespeare adaptations, Age of Bronze by Shanower, Beowulf by Gareth Hinds), or mass culture/pop culture phenomenon (The DC Big Book of Stuff, Image's Fractured Fairy Tales, etc.) stuff that explores folklore both familiar (Fables, etc.) and unfamiliar (Trickster Tales from Image, etc.)

Then some all ages stuff like Bone, Clan Aspis, Sandwalk Adventures, Archie collections, touching on super-heroes too like the Batman Adventures collections, etc.

Some biographies and historical pieces-March Vol. 1 and 2, stuff like Dignifying Science and the other science spotlight titles, Ho Che Anderson's trilogy about Martin Luther King Jr., 21 (the Robert Clemente story from Fantagaphics), the Saint and Boxers and American Born Chinese from Yang.

Then some random thoughts stuff like Trina Robbins from Girls to Grrlz exploring women in comics, and the Chicagoland Detective Agency all ages stuff she does now, to Hicksville by Dylan Horrocks exploring the idea of comics and what they mean to us. Modern stuff like McCloud's The Sculptor and some of the other stuff from First Second Publishing (from Paul Pope's Battling Boy to their Olympian gods collection to the Deliah Dirk adventures to Genius by Seagle and Kristiansen to Yang's Shadow Hero to Templar) to show what diversity and range comics can achieve beyond the big 2 niche of capes and cowls that most comic fans will never be bothered to check out.

Spotlight a few more female creators-Collen Doran's A Distant Soil, Linda Medley's Castle Waiting, and Carla Speed McNeil's Finder come to mind. Lumberjanes comes to mind as well though I have not sampled it yet.

Music & pop culture stuff like Hip Hop Family Tree, the Fifth Beatle, etc. that has a wider appeal than typical comics do is a must as well. Hit on some other pop culture milestones like Star Wars, Star Trek, Adventure Time, Walking Dead etc. that might appeal to fans of that and expose them to more stuff they can sample and try (without having to pay for it like they would in a comic shop-that's one of the beauties of a library system).

Make sure there are plenty of how-to books too, not just the McCloud and Eisner books on how to read that I mentioned previously, but things like How to Draw Comics the Marvel Way, some of the First Second Books like Mastering Words and Pictures, etc. so people who dig comics and might want to try their hand creating their own can find inspiration in some of the books.

Many libraries now lend digitally as well, and many publishers offer inexpensive digital copies that libraries can circulate as well, so I would suggest they look into that as a way to expand their collection without the cost of physical products to supplement (not replace) the physical collection they are building.

Point them towards places like the Cartoon Library housed at Ohio State University and other places that have curated these types of collections so they can get specific advice form librarians and administrators about how to go about building such a collection, and what pitfalls to expect and how to avoid them. etc.

-M

I mentionedt hem in my previous ramble...um I mean post.

-M

The Mid-Manhattan (5th Ave, and 40th St) public library has a complete separate room just for graphic novels and it contains a large amount of whats been already mentioned. it also has a huge amount of manga, Asterix and other euro collections. I'd also add to the list, which this library has, the hard cover collected editions of the Warren magazine classics  Creepy, Eerie and Vampirella published by Dark Horse and Dynamite, the huge Goon library editions, the Cerebus and Usagi collections, Ed Brubaker's various collections published by Image, the Groo collections, Rasl, Scott McCloud's Understanding Comics, Acme Comics Library by Chris Ware...I'm leaving much out but there must be a way to look up their inventory on line

Did anyone mention the Barks and Rosa Duck volumes that Fantagraphics is putting out?