I thought the Elite Four would be harder, so I overleveled Charizard. He ended up beating them almost solo, even against Lance’s Gyarados and Blastoise. I also forgot to replace Dratini with the Dragonite I had.
So, I think these are the best places for farming EVs. If you have any better options, I'd be happy to read them.
HP – Viridian Forest, as there are a lot of Caterpie there.
Attack – Rock Tunnel, as there are Machop there. Route 22, where there are Mankey, is also good (if you know any better places, please tell me).
Defense – Pokémon Mansion on Cinnabar Island, as there are a lot of Koffing there. Mt. Moon is also good because of the Geodude.
Special Attack – Pokémon Tower. I think it's obvious, since it's basically just Gastly.
Special Defense – Just surf on the water and you'll almost always encounter Tentacool, which is what you need.
Speed – Diglett's Cave, as there are literally nothing but Diglett there.
I hope I helped.