Based on personal preference, which comprises such completely arbitrary things such as how much I like pokémon designs from each typing, favorite pokémon from each type, and how much I actually use this type, my ranking goes:
Top
Water
Dragon
Fire
High
Psychic
Ice
Ground
Ghost
Mid
Electric
Steel
Flying
Dark
Low
Grass
Bug
Normal
Rock
Bottom
Fighting
Poison
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Courtesy of fr0q: http://img.imgcake.com/gencardjpgha.jpg
Courtesy of Pun: http://img861.imageshack.us/img861/4738/genesissaga.png