Tattoo collectors have different associations with the characters–and there are so many that you can find one perfectly suited to your tattoo style and subject.
Aside from Pikachu, popular Pokemon tattoo choices include:
Ash Ketchum
Bulbasaur
Charizard
Charmander
Blastoise
Snorlax
Of course, many of these characters can also combine with each other, and the pokeball, in killer tattoo variations.