With cheats
Feebas
On Diamond and pearl you don't really see many fire Pokemon or electric types. But really the rarest ones are legendarys. Also Garydos is a good Pokemon hard to get without super rod
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.
The most rarest Pokemon card is original set kahangaskhan. Worth more then $1000 a piece.
mew2 is the rarest
I would say an Empoleon, Azelf, or Palkia. Hey! it's only my opinion.
The rarest Pokemon ever in my opinion is probably Arceus since it is so hard to get. I like most pies except pineapple or cherry.
If your talking about the GBA games the rarest are Celebi, Mew, Jirachi and Deoxys until generation 4 (Pearl, Diamond and Platinum) Three new ultimate rares were introduced Shaymin, Arceus and Darkrai.
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.