If people do take the Dark Side Sourcebook ranking of him as "I" and therefore Sidious' equal seriously, at least we can say the strongest individual of the Sith species is comparable/equal to Darth Sidious.
__________________ Rebel by doing what gives you peace.
__________________ "Vader's pulse and breathing were machine-regulated, so they could not quicken; but something in his chest became more electric around his meetings with the Emperor; he could not say how. A feeling of fullness, of power, of dark and demon mastery -- of secret lusts, unrestrained passion, wild submission -- all these things were in Vader's heart as he neared his Emperor. These things and more."