With all those people playing Pokémon GO nowadays I was wondering which Pokémon of the first generation is the most resistant to all types?
You're in luck, as the Pokémon with the most resistances overall also happens to be a first gen Pokémon. The Magnemite evolutionary line boasts 11 resistances and 1 full immunity.
In terms of pure Gen 1, in a world where Steel, Dark & Fairy don't exist, and where Bug and Poison were super effective against each other for some reason, the most resistances any Pokemon had was 6. And the winners are:
Dragonite (Dragon/Flying) - 5 Resistances + 1 Immunity to Ground
Aerodactyl (Rock/Flying) - 5 Resistances + 1 Immunity to Ground
Rhydon / Golem / Onix (Rock/Ground) - 5 Resistances + 1 Immunity to Electric.
If you want to go a step further and take Dragonite out of the equation, only Aerodactyl and the Rock/Ground types have 4 Resistances, 1 Strong Resistance (1/4 Effectiveness) and 1 Immunity each.
Since Gen II and the introduction of Steel, Magnemite/Magneton, and as of Gen IV, Magnezone are the only Electric/Steel types, and as such are the only ones that have 9 Resistances, 2 Strong Resistances and 1 Immunity.
And, since Pokemon GO uses the Gen VI type chart with Gen I Pokemon, Magneton reigns supreme in the type department.
Magnemite and Magneton are the ones with the highest number of resistances, being 11: Normal, Grass, Electric, Bug, Flying, Poison, Rock, Ice, Psychic, Dragon, and Steel.