well it is either mewtwo or entai/raikou/soucuin and no for leaf but yes to emerald
Deoxys cause you can't get it.
By far, they have said Celebi is the hardest/rarest Pokemon to obtain outside of Japan.
Pokemon Ruby has Pokemon that Pokemon Sapphire does not and Pokemon Sapphire has Pokemon that Pokemon Ruby does not.
The rarest Pokemon is Munchlax, but you can find it only in the Valley Windworks honey tree.
treecko
well of course arceus but u can only get it on a wifi event
mew and mewtwo or possibly LAPRAZ it evolves from lapras at level 265 and is the hidden Pokemon 152!
my brother told me that it was on cinnabar island but i cant find it.
I cant tell you all the differences but I do recomend Fire Red version
deoxys and jirachi and there's even mewtwo in a secret place but that's for me to know and u to find out
Beat the elite for many times and let your Pokemon hold amulet coin to double the money.
always have tweneedle,then string shoot to lower speed,fury atack and toxtic.