Most powerful Jedi/Sith that Starkiller can beat?

Who is the most powerful Jedi that Starkiller could defeat in a random encounter? Who is the most powerful Sith he could defeat? Who would his rivals be in the era of Yoda, Mace, Obi-Wan, Anakin etc? Assuming all are at their canon peaks and they are fighting all out with full access to all of their abilities.

It be pretty high up there. I consider starkiller one of the most powerful and skilled jedi. I'm not sure how high the jedi list goes though.

Ventress

I know that he beat Vader, but I also remember Vader kicking his a@! A couple of times as well.