Gen 1:
Best: Vulpix
Worst: Rattata; honorable mention to Charizard, who isn't actually that bad but is massively overrepresented
Gen 2:
Best: Wobuffet
Honorable Mention: Girafirg (is either the best or the worst; I can't tell)
Worst: Murkrow
Gen 3:
Best: Gardevoir
Worst: Cradily; Feebas
Gen...