Strictly speaking, none of the terminology that is ever applied to comics as a medium is ever particularly accurate, so why be nitpicky about it? It originally came out in monthlies - so what? It reads as a “graphic novel” (a term I despise) now, so why does it matter?
As to the OP, I think it really depends on what you’re looking for. If you want more stuff in the vein of “Watchmen,” then “The Dark Knight Returns” is probably a good pick.
If you’re looking for comics, but don’t necessarily want super-heroes, “Maus” is, as others have noted, probably the top pick and the best place to start. Spiegelman is one of the major figures in comics, and “Maus” is his best and best-known work, and well-worth a read for anybody.
I guess the real question is, what do you like in other media? What kind of prose novels do you like? Movies? TV?
If you like “Lord of the Rings”-style fantasy, try Jeff Smith’s “Bone” or Richard and Wendy Pini’s “Elfquest” (though I’m not sure how easy it is to find in stores these days).
If you’re into sci-fi, you might try “Fear Agent,” which works as sort of a throwback to pulp sci-fi and 1950s “Weird Tales”-type comics. It’s all kinds of fun.
If you like more mainstream literary fiction, you might try Craig Thompson’s “Blankets” or Jason Lutes’ “Berlin.”
If you like historical stuff, try Stan Sakai’s “Usagi Yojimbo” series. Other than the fact that the characters are all rabbits, cats and other anthropomorphic animals, it’s a well-researched and hella fun Medieval Japanese adventure.
There’s comics to fit just about any genre you can name. Figure out what genres you want to read, then find a decent local comic store (or bookshop with a good comics selection), and go to town. If there’s not a good brick-and-mortar shop in your area, you can try www.cheapasstrades.com to help you find good prices on lots of stuff.