Someone who has been practicing the arts and continuing to practive for a long period of time. Also someone who has worked in various systems pulling details from each.
And then again the guy down the street who can teach you how to fire a gun might be just as good too.
Based on the choices of the poll.....
None of these are truthfully adequate for being a great instructor.
Having a great lineage is wonderful but doesn't mean you have the ability to teach what you have learned to others that may need different methods other than what you learned under.
Having Black Belts in several Arts is certainly an accomplishment assuming that you worked for them and know and understand well the complete curriculums of each system.... most of the time this is not the case. Even so, this does not translate into being a great teacher just being versatile.
Having a vocation such as a bouncer does give you certain realistic advantages that would be extremely useful in understanding what does and does not work on the street but once again this solely is weak.
Being a great tournament competitor is the same....... you may be excellent at what you do but cant relate a system or how you do it to others that may not have your gifted talent.
You need portions of all of the above plus the ability to teach and relate to others and work from a well formulated curriculum as well. It is easy to become an instructor..... However it is "NOT" an easy task to be a "great" instructor!