Don't think it really matters, most GCSE students have little choice with exam boards and they will just be chosen by the teacher/school, and most will end up with a mix of exam boards across their various subjects. Overall the content will be broadly similar or the same, there may just be slight differences in how they assess different parts of the syllabus and in the style of exam questions; such things aren't necessarily going to affect everyone in the same way, and some people may do better or worse with one style than another, so I don't think there is any general answer as to which is "best" because I don't think there is an overall "best", it just depends on the individual.
If you wish to you could look through the specifications for the subject(s) under consideration for the different exam boards and compare them, and also compare the format of past papers. Do note as well what, if any, coursework is required for each exam board, as it may vary. I don't think overall it will enormously influence your results though so if in doubt just pick one arbitrarily!