(I hope this post makes some sense)
In consideration for which Jin Yong story does the highest common level of central plain fighters emerge, we must define what qualifies as the highest level of central plain. First, this level should contain at least 3 fighters. As well, the fighters and the majority of their martial arts must be "from" the mainland. So, let's take DGSD for example. The general ranking of fighters may be as follows:
highest level: Sweeper Monk. --> He may be from the central plains, but this level only contains 1 fighter. Unqualified.
2nd highest level: Li Qiushui, Tonglao, Wuyazi. --> 3 fighters exist, but I don't think all of their martial arts can be considered from the mainland.
3rd highest level: Xiao Feng, Xiao Yuanshan, Murong Bo, Jiumozhi. --> Only Xiao Feng qualifies, so this level cannot be considered.
4th highest level: Duan Yanqing, Monks of Celestial Dragon Temple, Duan Zhengming, Xuanci and his fellow monks, Ding Chunqiu, Murong Fu, Su Xinghe, You Tanzhi, etc. --> Abbot Xuanci and a few of his fellow monks are roughly at this level, so finally this is the highest common level for DGSD's central plain fighters.
How about for ROCH:
highest level: for the sake of convenience, I am going to assume all the Greats are roughly at the same level. So, Guo Jing, Yang Guo, Golden Wheel Monk, Zhou Botong, Huang Yaoshi, Ouyang Feng, Hong Qigong, Yi Deng.
that was quite a few fighters, but which can be considered from the mainland as well their martial arts from the mainland? Guo Jing, Hong Qigong, and Yang Guo probably qualify (maybe some others too). So that's good enough.
Considering the highest common level of martial arts of the central plain, in ROCH we have Guo Jing, Yang Guo, and Hong Qigong; compared to Abbot Xuanci and his fellow monks in DGSD. Which do you feel is at a higher level? I'll leave that to you to ponder.
For HSDS:
highest level: Zhang Sanfeng, Zhang Wuji --> only 2 ppl, disqualified
(any level thereafter cannot match the level that the Greats are at)
lastly, I'll do one for Xia Ke Xin:
highest level: Shi Potian
2nd highest: Lord Long, Lord Mu
3rd highest: Zhang San, Li Si and their fellow disciples of Xiake Island.
4th highest: Xie Yanke, Bai Zizai, Priest Yucha, Abbot Miaodi --> this is the highest level in this novel