I still maintain it should be the witch king. We know that the 9 rings given to men empowered them and made them into Sorcerers and great leaders. And who is darker than the witch king?

The problem with being one of the blues is that yes they failed in their mission, but not till the 3rd age. They left middle earth in the second, and returned to valinor. Really that's what all of the wizards do, then they return in the 3rd.

Honestly what would be the most satisfying and likely acceptable to all fans is if the dark wizard became the witch king of angmar the leader of the 9 nazgul. This would make him a very very important character to try to derail.