I know everyone likes to talk about "chubby Pikachu," and...well, I'm one of those people who loves chubby Pikachu, too! But I'm gonna be honest here, I think RG's chubby Pidgey is the best sprite, along with Blue's chubby Nidoran F.
A lot of Pokémon from gen 1 have very round silhouettes (is that a Poké Ball, a Voltorb, an Electrode, or ... a Jigglypuff as seen from above?), and some of the ones that we think of today like Nidoran or Pikachu who have more unique silhouettes happened to be more rounded back in gen 1. It's super interesting to see how those things change. I'm generally not a huge fan of gen 1's sprites because a lot of them are really...frightening... But some are really fun, I must admit. Another one I love is Blue's Sandshrew, who looks a lot more like a pangolin in terms of his scale texture: