I prefer Pidgey which you can get in the beginning of the game. But the most powerful are the Legendary which are at The Powerplant (South of the rock tunnel), Victory Road (South of the Pokemon League) and Sea foam Islands (East of cinnebar). Dragonite is also an option, you can get a dratini at the waters in the safari Zone or at the game corner in celadon.
There is no best Pokemon. All Pokemon have type advantages. My best advise is to get a flying Pokemon, an electric, a grass, water, fire, and a rock/ground Pokemon (Geodude is good)
Flying type Pokemon are good against normal Pokemon.
Flying Pokemon can appear in many places. (I know this is not a good answer but there's nothing else I can think of.)
Swellow Salamence
the pokemon good against fighting are: flying ground fire (I think)
Sky
There are a lot of flying type Pokemon you can catch in leafgreen here are some good flying type Pokemon you should train remember you don't have to train these: Pidgeot, Fearow, Moltres, Zapdos, Articuno, Gyrados, Aerodactyl, Dragonite, Farefetch'd.
Flying And rock
You could possible find some zu-bats and if you are looking for a stronger pokemon there is a zaptos on the top floor
Altaria, Rayquaza.
no
Flying and Fire.