According to Sporcle, these are the most forgotten for each generation:
Gen I: Exeggutor Gen II: Qwilfish Gen III: Barboach Gen IV: Mantyke Gen V: Amoonguss
Surprisingly, Maractus is only 22nd-most forgotten among Gen V.
People just can't spell Exeggutor and Amoonguss, that's why they're at the bottom. The same may be true for Qwilfish, since it's pretty much infamous for being so forgettable, which has made it memorable.
I think Mantyke and Barboach are truly forgotten, though.