Clans are bonus specific. Some have attack multipliers, damage increasing, damage decreasing, posion effect, power increase, power decrease. Then others stop opponents abilities and bonuses, or protect the opponents abilities. There is no "BEST" clan. It's how you like to play the game at certain times of your carrer in the game. Now there are more used clans then others. Montana, Rescue, Uppers, Nightmare, and Pirhanas are more seen then some others.
I like to use Gheist, Montana, Skeelz, and Pussycats. My favorite four and there are others that I use because of the collection, but I don't play well with them. I'm use to how the other clans work with each other and how they react with other clans.