I say Team Rocket.
They owned their own game corner. I'm gonna go ahead and say that means they've got plenty of money.
They've actually killed a Pokemon.
They completely took over Saffron City.
Their BOSS, the head guy and infamous criminal was able to be a Gym Leader.
Three years later, the underlings were so dedicated to the cause they tried to bring the team back together and bring back Giovanni.
They managed to drive hundreds of Magikarp crazy and force them to evolve.
I think that's about it. Oh wait. I think I might've overlooked one minor thing. Mewtwo.
While Cyrus tried and failed to summon a legendary Pokemon so he could use its power for a new world, Giovanni used his own power($$$) to create a powerful new Pokemon.
Sure they eventually failed, but at least they got things done. All Team Galactic did was set up a bunch of buildings and steal energy. Although I will admit it's pretty cool that they blew up a lake.
I'm seriously gonna go with Team Rocket