Use Good or Super Rod and you can catch it at route 9 or 10
There is no best Pokemon to catch, as you train each individually.
You should be able to fish and catch a poliwag, that will evolve to a poliwhirl at level 25. Bulbapedia says that Poliwhirls can be caught at: Routes 22, 28, 30, 31, and 44, Ecruteak City, Violet City, Mt. Silver, and Viridian City (Surfing) you will have the best luck fishing or surfing
the best Pokemon you can catch in Pokemon pearl is heatran
the best Pokemon is kyogre
mewtwo
Palkia
You can catch Moltres at Mt. Ember Peak in FireRed.
All of the starter Pokemon are the best
Donkomon
the wild Pokemon modifier code. you can catch an Pokemon with it.
there is no best spot
master ball