the only problem that i have with Vader is - although he was a very bad man for a long time - he showed he still had goodness in him. And ultimately that goodness overcame his evilness in the end (like you alluded to Benzo). Hence, you can't really give the title to him out of principle because in a small way - he was a "good" guy - at least he ended up as one.
Us viewers were rooting for him in the end. rooting for a villain. fail.