Who were the Ringwraiths?

[A_Brandybuck]

Men proved easier to ensnare. Those who used the Nine Rings became mighty in their days, kings, sorcerers, and warriors of old. They obtained glory and great wealth, yet it turned to their undoing.

-Of the Rings of Power and the Third Age

This quote, that Kuruharan has posted already before, does only imply, that they became mighty in their days, kings, sorcerers, and warriors of old after they had received their Rings.
Consequently it could be, that some of the Nazgûl were normal Men, with no title or noble origin. The Rings made them mighty, it must not be, that they had been mighty before.
That let to the question, on which criteries Sauron has chosen the Ringbearers. Surely were most of them Númenórians, they are supposed to be more powerful as normal Men because of their origin. That there are at least some Númenórians, shows this footnote of Letter 156:

There were evil Númenórians: Sauronians, but they do not come into this story, except remotely; as the wicked kings who had become Nazgûl or Ringwraiths.

And that not all are Númenórians is shown in the title of Khamûl: the Easterling.