Cell. In one episode, King Kai and Goku talk about him and Kai actually says Cell is stronger than Frieza. Also you have to take their abilities into retrospect. Freiza can transform to improve his power, so can cell. Cell has Freiza's abilities and his own as well. But if it's cells first form, Freiza's stronger, if it's his imperfect form, their pretty much even, but in his perfect form, there is no contest. And Goku couldn't even beat him, even with a years worth of training in super saiyan, he still couldn't beat him, and also the entire Z-fighter crew couldn't beat him until gohan got angry and went berserk. So bottom line, Cell could take on four super saiyans, Freiza couldn't even take on one. Need I say more?