Name the most popular Pokemon anyone remembers the existence of

It has to be Pikachu. Even the totally disinterested, uninformed non-gamer probably knows Pikachu, if not through exposure in other games and media, then overwhelming cultural zeitgeist. It is to the point that your grandmother might think the name of the franchise is Pikachu, rather than Pokémon.

Special mention goes to Eevee, who perhaps serves as a secondary mascot and seems to be particularly popular among women. It often appears on merchandise alongside Pikachu. Its various evolutions also mean that even if its base form does not appeal, one of them might.

Charizard of course deserves special mention, too. And Mewtwo, being the "final boss" of the original games, the antagonist of the first movie and a competitive all-star.

During Sun and Moon I noticed that Mimikyu also seemed to attract quite a following, quite fittingly given its lore, with many non-Pokémon fans I saw with merch stating that they preferred this new look for Pikachu.