Pokémon FireRed and LeafGreen/Route 19

'''Route 19 is a predominantly north-south water route in southern central Kanto. To the north of this route is Fuchsia City. Its south end is at Route 20 east of the Seafoam Islands.'''

Route 19
Route 19 is just south of Fuchsia City and is full of trainers and Pokemon. Leading to the Seafoam islands, it's the long way towards Cinnabar Island. Head to Route 21 to take a short path to Cinnabar, or start out on Route 19 for experience and money.