Water is the most common. Ghost is Least common.
UPDATE: As of generation V, Dragon is now the least, Ice is second least and Ghost is third.
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The most common Pokemon is Pikachu.
Find a Flying-type Pokemon and catch it. There are plenty of them in each game and many Flying-type Pokemon are common Pokemon, so you shouldn't have trouble doing so.
The most effective in rock type Pokemon is water Pokemon ofcourse.
The most effective type to battle with against a normal type Pokémon is a fighting type.
Water with 109 different species.