Pokemon yellow
The rarest Pokemon is Dialga
there is no rarest Pokemon you have to train and train and including shinies there is no real rarest Pokemon at all.
mew2 is the rarest
Arceus and Deoxys are the rarest Pokemon to date
No, you do not need cheats to get the rarest Pokemon. The only way to get the rarest Pokemon without cheating is to go to a Nintendo Event, held every few years or so, and download the rarest Pokemon there. Otherwise, you would need cheats to get the rarest Pokemon.
Pikman or Pokemon The Rarest is mewtwo
The rarest is stantler.
zekrom
By far, they have said Celebi is the hardest/rarest Pokemon to obtain outside of Japan.
Black and yellow
That would have to be the Pokemon Arceus