The "best" usernames are already taken. However if you have a creative mind you'll be able to come up with something neat. Many people find that composing usernames out of 2 or more words works best. Usually people in the forums are happy to address you with a nickname rather than your entire username.
profilebrand.com
Here's a link to a good name generator that provides character names for gaming.
Gaia Online has some pretty good manga avatars
Softball_Savy
ihaveabigbutt232
Good usernames for Tumblr should be unique and creative. Make a username that is an expression of who you are and your personality.
ilovesweets23
junior
Good usernames for dancers should convey how much they love to move their body. Some examples are joyfuleaper, gracefulone, fluidlimbs, movingart or dancingdamsel.
Gaia online is one...
SexyLexi, Lexiboo, lexiboo22, l_e_x_i, lexilove, lexi_love.
MayaMarvel MayaMoonlight MysticMaya MayaMagic