As far as evil goes, Voldemort wins hands down. Remember that Vader first turned to the dark side because of his desire to save Padme, hence proving that the road to hell is paved with good intentions. And he never fully turned. Sure, he was evil bad guy up until it was Luke vs. Palpatine, but then something clicked and he sacrificed his own life to kill Palpatine and save Luke, turning his back on his master and the dark side as a whole.
Voldemort, on the other hand, is nothing but evil. The only reason he became the Dark Lord as opposed to Tom Riddle was for his own benefit. He wanted to become the most powerful sorcerer of all time, not to mention the whole immortality thing. And when you look at what he did to himself and others to attain that, there's no way even the slightest spark of light exists in him.
Now as for who would win in a knockdown dragout, that's a little closer, but I'm still putting my bets on Voldemort. Sure, Vader's got <insert Force ability>, but Voldemort has a greater array of abilities at his disposal thanks to his magic. Plus, as proven during his involvement with Luke, Vader can hesitate. Voldemort, on the other hand, lets nothing get in the way of his goal of immortality and domination of the wizarding world.
Which makes me wonder, is there a spell in the wizarding world that allows one to conjure an energy blade from one's wand? Rowling never mentioned such an incantation, but if it does exist, then Vader's lightsaber has no advantage.
_________________
"Yeah, so this one time, I tried playing poker with tarot cards... got a full house, and about four people died." ~ Unknown comedian
Happy New Year from WP's resident fortune-teller! May the cards be ever in your favor.