I'm with like almost everyone here, Xenmas would wipe the floor w/ Vader! ><
Just because everyone else says something you automatically agree with them without question, in the words of Darth Vader "you are a weak minded fool not worthy of the force", saying Xemnas is able to beat Darth Vader is like saying an ant can beat a Dragonball Z's Kid Buu who can blow up planets with his finger tips.