Vegeta. Biggest threat of the Saiyan Saga.

"I do not fear this new challenge, rather like a true warrior I rise to meet it"

 The biggest threat of all during the Saiyan Saga was none other than Vegeta. The Saiyan Prince had more control of his power than anyone else Goku and his friends had faced thus far. It is honestly incredible that Goku managed to defeat Vegeta when he originally invaded Earth with Nappa. If he had failed, all of humanity would have been destroyed.