Great thread, CConn
I think the key phrase in your proposition is "widest representation". Ask this question of a Spider-Man fan, even while aware of other areas in the comics industry, they might only recommend Spider-Man books (or just Marvel books, or just Superhero books, or maybe they;ll throw in one other acclaimed book they know of like Sandman or Maus)
But if I were gonna break down what would qualify as what would most widely represent the medium in only five books (single volume trades or graphic novels, I'd look at it like this:
1 Superhero title - vilefy them or worship them, they are the predominant representation of American comic books
1 Manga title - love it or hate it, the Japanese comics market is more than 10 times larger than the American market. Any list that doesn't acknowledge this is inclomplete and not representative
1 Non-fiction title - a vastly underrated and underused category for comics, I don't think a list comprised of five pure fiction titles would be appropriate, nor accurately display what the medium is CAPABLE of, if not what it is.
1 Mature Readers title - to show the other side of American comics
1 Retro title - to show the medium's roots
At least one title is allowed to be depressing as hell, at least one title should be uplifting and fun.
Superhero title - WAAAAY too many to choose from. Invincible is a current favorite and undoubtedly tons of fun. A Spider-Man book seems like a good way to go, but which ones? Same with Superman and Batman (Although Year One, Long Halloween, and DKR all come to mind). Watchmen, MiracleMan and other Alan Moore titles all come to mind. Hell with it, I'll cheat and choose easily the best and most diversified collection, an anthology: DC Universe: The Stories of Alan Moore.
Manga title - I could debate this one - but is anyone really gonna argue with Lone Wolf and Cub?
Non-Fiction title - Maus is the frontrunner given it's Pultitzer Prize winning status, but underappreciated books like Blankets, American Splendor, PyongPyang, or Torso come to mind. Understanding Comics is an excellent example of
referential comics, an examination of the medium, but I don't think it's a good representation of the medium itself.
Mature Readers title - So, so many to choose from (most with the words "Vertigo" on the front). Most everything with Alan Moore or Neil Gaimen's name on it, brilliant works by Ellis and Vaughan like Planetary or Pride of Baghdad. Miller's Sin City and Ennis' Preacher are brilliant but not easily summed up in a single volume. This one's tough.
Retro title - I could debate this, I could insist on something from Stan and Jack, but no - this one belongs to Will Einser's The Spirit.
and those're my thoughts on the topic. Thanks CConn