I think Valk seems more powerful because he uses more Force attacks and has a greater knowledge base, but Zonakin's domination of Dooku is pretty impressive if less flashy IMHO.
So Valk oneshots Arcann with lightning and it looks cool but KF Vader would just tool him in sabers with one hand, not as fast but probably requires a comparable amount of power.
How that translates into resisting Valk's attacks, Im not sure. Dooku's Force powers were a joke to Zonakin, for example.