The question is not which martial art would be the one most likely to win in a fight, as that is largely dependent upon the fighter, but which is the most profound, sophisticated, and advanced martial art in DGSD...the Jin Yong story known to have some of the most incredible martial arts skills in the author's canon.
Other great martial arts skills such as the Mo Yung Family's skills and the Beggar's Union's Hong Lung 28 Palms are also great, but the above three arts are a cut above those.