I agree that Watchmen has had the widest audience and popularity. Persepolis is the one I forgot.
I guess when I am recommending one graphic novel I just do t want to stick at "the most influential one" although to be honest that's not a bad starting point. Watchmen encapsulates a lot.
Blankets and Stitches. Two different authors yet very similar feels. Very different from watchmen et al of course. Realistic, quasi autobiographical.
Right now I've been reading. swamp Thing. Amusing considering I am also reading Swamplandia!, I appear to be on a sort of kick.