Though it may seem counter-intuitive because they're the first few Pokémon in Generation 1's index listing, Rhydon, Kangaskhan, Nidoran♂, and Clefairy are actually the hardest of the original 151 to get via the extended Mew/Trainer-Fly glitch: It's impossible to get a Pokémon with a Special stat of 1-4, so in order to encounter those four you'll need one with a Special stat of 256-259.