FireRed has the same legendary poke'mon as LeafGreen. You can capture:-
Mewtwo in Cerulean Cave
Articuno in Seafoam Islands
Zapdos in the Power Plant
Moltres in Ember Island
Lugia in Navel Rock
Ho-oh in Navel Rock
Suicune roaming around Kanto if your starter is Charmander
Raikou roaming around Kanto if your starter is Squirtle
Entei roaming around Kanto if your starter is Baulbasaur
Jirachi roaming around Kanto
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No
No legendary resides on the islands a rare Pokemon resides on seven island in the tanoby chambers the Pokemon is unown.
migrate from firered and leafgreen
If you're talking about the legendary Pokemon Articuno, you have to trade it from FireRed or LeafGreen. If you're talking about Delibird, you have to trade it from FireRed.
Well there is no such thing as the best legendary, everybody has his own tastes. The higher leveled legendary you find here is Mewtwo.