I would say Vel'Koz.
He is the oldest surviving voidborn, and he used his lifespan to search knowledge in the most effective way. He is also one of the first of the voidborn. He is/was observer of the Watchers. He has a special place in the Void hiearchy and definitely not a simple voidborn.
I will say Vel'Koz is far more cunning than Ryze, and Ryze is fragile, a glass canon. For that reason he would likely die even before fight started. Not because Ryze is weak but because Vel'Koz is an "otherworldly" supernatural being even among supernatural beings...