Palpatine is officially the most powerful Sith, and by powerful, I don't mean forcewise, I mean everything. Skill, ability, saber combat, intelligence, etc., and that is a fact. Maybe I should have been a bit more specific, yes.
And Mace Windu defeated the Emperor thanks to his mastery over Vapaad, and to his Shatterpoint ability, not mere luck.
Well... By ROTS Sidious is already established as the most powerful ever. If he gives DE Sidious a pretty good fight, as you say, then he logically gives the lesser version an extremely difficult one, which is hard to believe... I say he gives ROTS Sidous a good fight, but is defeated without that much difficulty by his DE version. Though, not easily.