WCY taught YD the "pre-heaven skill" which with his "yi yang zi", he can easily defeat OYF. Throw in the fact that he is the only one that studied the 9 yin zhen geng, how is he not considered the best of the greats?
Also WCY's "pre-heaven skill" is the highest level of quan zhen internal cultivation where not even ZBT knows, this alone should put him on par with WCY right?