Windu's only superior to Palpatine in terms of lightsaber skills. In terms of Force power, Palpatine ~ Yoda > Mace. Even that's debatable due to Vapaad being an excellent counter to Dark Side users. Without his utilisation of Vapaad, Mace may have ended up the same as Fisto, Kolar and Tiin did.
Vader's loss to Luke was likely due to him holding back due to being conflicted in his ideals, which Snoke even cited as the only logical way Vader could have lost in one of the novels.