Marvel, though DC has a solid showing.
1) The most popular villain: Joker, probably.
2) The most evil villain: Darkseid, I think. Marvel probably has a rough equivalent, but Darkseid is put to greater use and used for greater effect.
3) The most powerful villain: ? Both Marvel and DC have existential threats to their respective universes. I'll leave it up to Death Battle to figure out which would come out on top, but it's probably a tie.
4) The best written (rounded) villain: Magneto, by several light years. Almost every other (of the better written ones) looks half-assed in comparison.
5) The villain with the best character design: Doc Oc.
6) Largest number of great villains: Marvel; Batman is the only DC heavyweight with a good rogue's gallery. Marvel has a good one for Spider-Man (almost as good as Batman's), X-Men and the Fantastic Four. Also, Marvel does a better job with putting its heroes on the side of villainy through moral ambiguity. DC's attempts seem fairly black-and-white even when it tries to be nuanced.
