depends on what your weaknesses are.
I don't draw Pokemon but i am an artist. i"m basing my answer off of the pokedex pictures.
I always mix up the swirl direction on Polywag, Poliwrath, and Polywhirl.
If you want to be exact to detail ,I'd say it was Gastly, koffing, Moltres, Altaria, or Groudon.
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spiritomb, because it has no weaknesses
By far, they have said Celebi is the hardest/rarest Pokemon to obtain outside of Japan.
Not including the legendaries and special pokemon ( ones that you can only catch once ) feebas is the hardest and rarest pokemon to catch
Beat the pokemon league but they are the hardest to beat in the hole game hope I helpd