well in my experience it would have to be arceus in Pokemon diamond. It took me an hour in the battle to weaken it without defeating it.
Deaoxys.
the hardest Pokemon to find is regigigas you can only get him after finding all the other Pokemon and you can find him in the hippodow temple
Magikarp
latias
A fleeing legendary pokemon
feebas
darkrai and arcus
The easyest Pokemon is scizer and the hardest Pokemon is milotic.
Mesprit & Cresselia (legendary)
spiritomb, because it has no weaknesses
Pokemon Rocks Hardest
I chose Chimchar. it has been proven that: Chimchar is easiest pokemon to play with in the game Piplup is medium hardest pokemon to play with in the game Turtwig is hardest. This has been decided on the gym leaders, trainers and wild high level pokemon. Hope this helped!
By far, they have said Celebi is the hardest/rarest Pokemon to obtain outside of Japan.