Although most of the Greats actually achieved around the same level of martial arts, if you had to, How would you rank the Greats' martial arts at their peak? For the sake of consistency, martial arts in this context is defined as "the combination of theoretical knowledge, experience, and actual combat performances." Also, for the people who specialize in weaponery, we assume that they do not have their weapon all the time, although at the Greats level weapon does not really mean much, or else they wouldn't be the Greats.
The characters who were mentioned but never appeared are denoted in (parentheses).
1. (Demonic Swordsman Dugu Qiubai)
2. (Huang Shang, creator of "9 Yin")
3. (Central Divinity Wang Chong Yang)
The 3 above characters were legendary masters who were either revered by later generations, or produced unparalled martial arts during their time. How they would compare with ROCH Greats is debatable, but due to their achievements and reputation, my impression is that they should not be inferior to any Great, only better.
4. Guo Jing
his martial arts saw a steady increase from his teenage years, to his encounter with Ouyang Feng, to his encounter with Golden Wheel Monk + 3 Mongolian warriors. Each time he demonstrated more refined mastery AND effective battle performance compared to the previous one. If this growth pattern continues, he should outmatch any of the Greats by end of ROCH.
5. Yang Guo
could not defeat Huang Yaoshi or Zhou Botong, but it's not like he was trying to. He was engaging in friendly competitions with them, and these are people that he respected. On the other hand, it was quite clear that the empty-handed Yang Guo is still better than weapon-wielding, post-10th level "Dragon Elephant Sutra" Golden Wheel Monk. And when he could utilize Sad Palms to the fullest extent, he would win. Despite being better than GWM, Yang Guo has the disadvantage of being one-armed, and the technique that gave him the most trouble was the Left/Right technique. Since Guo Jing's power is not inferior to Yang Guo AND Guo Jing knows this L/R technique, Yang Guo would have a major disadvantage.
6. Zhou Botong
was slightly better than the rest of the Greats all along.
7. Eastern Heretic Huang Yaoshi
8. Southern Emperor Yideng
It was mentioned that Northern Beggar and Western Venom improved greatly between LOCH and ROCH. I see no reason that Southern Emperor and Eastern Heretic would not also improve greatly, and they underwent 16 more years of improvement compared to their 2 deceased peers. Although their stamina will not match their 2 peers', their refinement could have surpassed the other 2 by a long shot. I rate Huang Yaoshi slightly higher because he became a Great on his own, while Yideng became a Great with the help of his family kung fu.
9. (Lin Chaoying)
At the beginning of ROCH, Qiu Chuji told Guo Jing that Lin Chaoying would be one level superior to the Greats in terms of martial arts. We don't know if Qiu was referring to the Greats at Lin's time or at present time, but we do know that Lin Chaoying was somewhat inferior to Wang Chongyang BEFORE 1) Chongyang developed Quanzhen Sect, 2) Chongyang absorbed the essence of 9 Yin. Lin also developed the Jade Maiden style, so she should have continued to improve over the Greats. It's quite debatable where she lands in this ranking; I simply took the middle path of Qiu Chuji's words: if Lin was better than the Greats up to ROCH, then she should be superior to all of them at the end. But if she was only better than them during her time, then they had 30+ years to improve, and they should have surpassed her. At the very least, I imagine that she would overcome Golden Wheel Monk (see below for his weakness).
10. Hong Qigong
11. Ouyang Feng
I rate Hong Qigong 0.1% higher because his kung fu base was more orthodox, which tends to become more refined over a long period. Although he would not carry Dog Beating Stick all the time, he can probably substitute any tree branch to defeat Ouyang Feng (Yang Guo figured that Ouyang Feng would have lost to Dog Beating Stick in real time). But in a life-death situation, Ouyang Feng might fight better.
12. Golden Wheel Monk
should be slightly more powerful than the Greats, due to his superior stamina. BUT, and this is a big BUT, since he showed that he is a mediocre fighter with cowardly attitute (getting scared frequently), I moved him below the Greats. I mean, can we possibly imagine Dragon Girl forcing a Great to complete defense, and even injuring him?! Granted, that was before Golden Wheel Monk reached 10th level of the Sutra, but the (2) Greats also improved in the mean time. Judging by his last battle with Yang Guo, his cowardice didn't get better. As soon as the Greats figure out his weakness, they can try to confuse him with dazzling technique. He would have no chance against Ouyang Feng for one, Lin Chaoying for two.
13. Qiu Qianren
Feel free to share your opinion. Note: I did not include any HSDS Greats, but you are welcome to incorporate them into your ranking if you would like.