Once again, I'll preface this with a disclaimer, this is only my opinion and shouldn't be taken to seriously and by saying this I am in no way putting Marvel down this is just a thought that entered my head recently. 
So, it's often accepted that Marvel's heroes are much more relatable than DC's heroes. Now this is less true now than it used to be, but for the most part, DC's superheros are kind of larger than life and icons, where as Marvel's are much more down to Earth and feel like they could be people you know personally. Basically it comes down to Marvel heroes like people make mistakes because they're "human". DC's heroes do make mistakes now and again, depending on the story but for the most part they are reactionary characters. For Marvel bad stuff happens because of the mistakes the heroes make (again not always but often). This is why DC's villains come out on top to me. When a bad guy "wins" in Marvel it's because they pressed an advantage granted to them because of a heroes short comings, but when a villain "wins" in DC it's usually because they've just genuinely beat their respective hero. 
Let's look at it this way. I've been watching the Spider-man show from the 90's since it's on netflix. The series of episodes that involved the vampires come to mind as a good example. Spider-man at one point realizes that the vampire who's been hurting college students on his campus is a fellow class mate. He sees this class mate transform back to a human in the sunlight. Then he leaves the guy on the middle of campus because he's got to go work on something for a class. He assumed that the transformation was done for good (kinda what was said in his inner monologue). Thing is, it wasn't a one time transformation, the student became a vampire again the next night and continued to hurt college kids. The villain continued to be successful, yes, but not because he was better than Spider-man but because he took advantage of a mistake Spider-man made.
Now on the other side of this, let's look at one of my favorite stories "Riddle Me That" from Legends of the Dark Knight. At the end of this story the Riddler has put Batman into a situation where he was forced to break into a museum  to steal something for him. Batman tipped the situation in his favor and forced The Riddler to try and run away with the object in question. The Riddler runs into a trap set by Batman involving the son of one of Riddler's victims. The son beats the Riddler up quite severely, the cops show up, and Riddler is put into a cop car to be sent away to Arkham yet again. Batman and the commissioner look at the object Riddler tried to steal (a map that's been folded up till this point) and inside they find a green question mark. Next we see Riddler in the back seat of a car being driven by the same guy that beat him up earlier.  He beat Batman not because of any mistake on Batman's part but because he was able to top Batman when he was playing at the top of his game. 
So that's my argument, DC villains are better because they are able to top their respective heroes on their own merits, they don't need to get lucky and press a mistake on the good guys part.