Hmm...
Someone quiet and respectable, who wouldn't particularly mind subservience. Someone who is competent in a wide variety of fields, who is self-sufficient and unquestionably loyal. When I rise to power, they would need to recognize that my authority is not automatically theirs by extension. They would also need to be able to fill in as my aide-de-camp if the need should arise.
Oh, and whoever they may be, they'd have to be easy on the eyes. I am going to keep them around for a while after all, and I think my perfect slave should have some good looks on top of everything else.
You see, if I'm going to have a slave, I would like no regrets associated with them down the line. If they fail to meet the criteria, then I simply won't take a slave.