It depends on your lv probably the toughtest place you were in.
If your reffering to 'Pikachu' (#25 in the Kanto and national pokedex.) you can find it in a lot of places. Your best bet is going to 'Seribii.net' In Pokemon FireRed/LeafGreen you can find it in Viridian Forest.
the best bird Pokemon on leaf green is pidgeot
i am personally a fan of exeggutor
The original startes; bulbasaur, charmander and squirtle are the starters (and in my opinion, they are the best).
i like zapdos but i depends. snorlax and blissey are also preety good
train it at the Pokemon league
Beat the Pokemon league and fill the pokedex. Raise a team, and challenge all of your friends to be the best.
dragonite is the strongest dragon there is in leafgreen
I think its moltres
I guess Zapdos
the best Pokemon are mewto and the rarest Pokemon ever mew.
pikapoo
Gyarados
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Charmander
MunchingOrange
There is no best Pokemon to catch, as you train each individually.