I have joint favourites of Squirtle, Sceptile, Altaria, Azumarill and Gengar. But if I HAD to pick just one I think it has to be my purple embodiment of a genderless void

a purple monster is flying through a blue sky with clouds

Alt: This image is a GIF. The Pokémon Gengar is flying through a blue sky with clouds before turning around and throwing a purple coloured shadow ball towards the camera