Togepi and Marill
the only deciding factor in choosing Pikachu and Eevee were popularity. Pikachu was of course the mascot for Yellow Version, and people love Eevee for its many evolutions. For Gen II, Togepi was pushed as the main mascot for the region, but then Marill also got quite a bit of attention due to the 'Pikablue' rumors and then its addition to HGSS as your friend's partner. I would say Pichu over Marill, but Pichu is just the baby form of Pikachu so I dont think people would be as excited