Sidious. Dooku lost to Anakin specifically because his fighting style was weak against Anakins. Sidious doesnt have that problem, plus at that point he is still stronger than Anakin in the Force so there no way Ani can beat Sidious at this time. Had Vader even 6 months to a year as a Sith without Mustafar injuries I'm sure he'd beat Palpatine quite handily.