I'm not sure if this goes into ZWJ thread or deserves a separate one, but if this belongs in the ZWJ one, my bad.
I think it's safe to say that JY has established formlessness as seen in taiji jian or DG9J as the theoretical ultimate of martial arts. However, it's one thing to establish a theoretical limit, than to achieve it.
My question is: Are there conclusive evidences from the novels that establish a clear superiority of formlessness over an equally skilled fighter with superb techniques?
My gut feeling is no, until someone proves me wrong of course.
We have Dugu, who's all smoke and flash, with nothing substantial than boastful inscription and set of teachings.
He had two disciples, so to speak.
Yang Guo was undoubtedly good, yet he at his height never fought Guojing and his 9 yin techniques and 18 dragon palms. Thus no clue if YG's heavy iron sword and his Dugu teachings could've defeated a 1st rate technique.
Linghu Chong was good too, but he hardly had any decent competition, and when he did find a suitable opponent, he had to have the help of many 1st rate fighters to win. Really, in SOD, other than KHBD and DG9J, there's nary a 1st rate martial arts, IMHO. Although you could say the taiji jian was 1st rate, but the practioner wasn't a 1st rate fighter, not like Dongfang. Nothing conclusive again
ZWJ and ZSF were the other masters of formlessness, yet HSDS is a hardly a measure of taiji's effectiveness, given that the two were so obviously superior to everyone else. Nothing conclusive again.
It's somewhat odd that formless martial arts were never truly tested in JY's novels, to my knowledge. It'd been different if the situation was like in LoCH, where greats of roughly similar powers battled each other, but such situations never happened. the golden monk was hardly a worthy opponent to push the iron sword or Dugu's teachings to its limit.
My point, I guess, is that the formlessness is a wonderful theory and potentially the ultimate form of martial arts, but it's far from clear that it's in fact humanly possible to reach that potential.
For all we know, a best technical martial artist will beat Dugu because Dugu himself never reached that plateau.