deoxys and jirachi and there's even mewtwo in a secret place but that's for me to know and u to find out
The rarest is stantler.
By far, they have said Celebi is the hardest/rarest Pokemon to obtain outside of Japan.
Deoxys cause you can't get it.
the rarest Pokemon in Ruby Sapphire and Emerald is Feebas
The rarest you can get is groudon who is a fire-machine type pokemon.
The rarest Pokemon is Munchlax, but you can find it only in the Valley Windworks honey tree.
Pokemon Ruby has Pokemon that Pokemon Sapphire does not and Pokemon Sapphire has Pokemon that Pokemon Ruby does not.
groudon
well of course arceus but u can only get it on a wifi event
pelipper
If you have Sapphire, it's Latias. If you have Ruby, it's Latios. And if you have Emerald, you get to choose.
mew and mewtwo or possibly LAPRAZ it evolves from lapras at level 265 and is the hidden Pokemon 152!