The choice of a dojo is a very personal one. Everyone has their own path of learning, their own goals and expectations. The “best” dojo for one person is likely not the same as the “best” dojo for the next person.
If one is lucky enough to live in an area that has several, visit all of them. Get permission to watch some classes and observe how classes are taught, and how the students and instructor(s) relate to each other. Is there a sense of mutual respect? Is there tension and fear? Does the atmosphere seem to promote growth? Most importantly, are you comfortable there? Try to visit while the chief instructor is teaching, but also try to get a feel for other people who may be teaching. See if the teacher appears to respect the students, and that they respect him/her.
Talk to various students and ask their opinions meanwhile noting their demeanor. A good sign is if either before or after a class which you are observing, a senior student or instructor comes over and talks to you. If you are ignored, it may not be the kind of dojo you are looking for.
Remember, this is where you hope to be spending a fair amount of time, and you want it to be a positive and rewarding experience. One final question, also very important, is simply “Do I like this dojo and these people?”
Beware of any dojo that tries to pressure you into a quick decision or is averse to letting you watch a few classes.
While the chief instructor’s rank may be somewhat important, be wary of inflated qualifications. Although an affiliation with a national or international organization is no guarantee of quality (and some teachers are not affiliated at all), it can be a useful indication. Promotions given by non-affiliated dojos are not likely to be recognized anywhere else, and this may well be a consideration.