I'm going to vote Darth Vader. Hannibal did seem CRAZIER, but was he truely more evil? Hannibal was crazy, he couldn't feel anything while Darth Vader full well felt and knew the evil he was doing, and still did it. Darth Vader killed Billions more people then Lecter did. Hannibal commited evil acts because they made him "feel good". Darth did it for the sake of being evil. If Hannibal was given the choice to torture or save a stranger, he would torture him without even realizing that he could save him. However, if Darth Vader was given the choice to save or torture a stranger, he would choose to torture him EVEN THOUGH, he full well knows that it's in his power to save the stranger instead. Knowing that you CAN do good, and purposly doing the opposite of it is more evil, in my opinion, then doing evil without even knowing that you can do good.
Also, one can argue that Darth Vader was more evil because of what he did with what he had. Darth Vader planned out his evil acts and did them accordingly to what plans he had. When he gained the power to blow up planets, he damn well did it! As for Lecter, well, if Lecter ever had the power to blow up planets, I bet he would just stick with killing his victims one by one. He's a crazyfuck that doesn't plan big things ahead like building giant death machines to blow up planets or gain ultimate power in the entire freaking galaxy where evil can be much more widespread and effect many many many many MANY more people, Aliens, Droids, and any other form of Organic or Inorganic life. In short, Lecter is more Chaotic while Vader is more Evil.