After or before Brock's gym battle?
If is before, the viridian forest is the best place to train, eletric, flying, fire, normal types, psiquic, but also work with any other with not much easier or harder difficult results.
If after him, there is alot of place to train, The routes in Cerulian city, are perfect for training, flying, fire, eletric, rock, normal, psiquic, ground. There are alot of Pokemon weaken to thoose types here, pidgey, caterpie, oddish, belsprout, nidoran, cacuna, etc.
Mount mon is best place early on to train ground, eletric, grass, psiquic, fighting types due to have many geodude, zubat.
in early game, thoose are the best.