What i want from my main villain is someone that is really evil but admirable in the same way (something Malak wasn't). Not evil itself incarnated, but someone who has a personal agenda behind he/she is evil. Maybe due to manipulation of outher forces, or from own experiences that lead him/her down the path, someone that you really want to kill at the end of the game, not just a obstacle in the way for you to complete the game.
I want my arch nemesis to be someone in the vain of Darth Vader/Sephiroth, something devs seem to forget these days.