There's a Gu Long ranking somewhere here. Here's my Jin Yong ranking to start a discussion. There's probably some disputes about how I rank them. I'll omit those who didn't appear such as Dugu Qiubai. I want to focus on those that made an appearance and actually faced combat in the novels.

1. Sweeper monk

Defeated numerous legendary figures easily. Enough said.

2. Dongfang Bubai

Even the combined effort of Ren Woxing, Linghu Chong and Xiang Wentian was no match against him. He had the upper hand and would have won if not for the distraction caused by Yang Lianting. To hold his own so well despite being outnumbered by high level opponents is no mean feat.

3. Zhang Sanfeng

The founder of Wudang had no match, even when he was seriously injured.

4. Zhang Wuji, Shi Potian

Zhang Wuji mastered many abilities:

Wudong Long Fists
Seven Injury Fists
Cloud Ascending Steps
Nine Yang Technique
Great Heaven and Earth Shift
Dragon Claw Hands
Taiji Fists
Taiji Swordplay
Holy Flame Tablets

Likewise, Shi Potian picked up many skills:

Shaolin inner energy cultivation
Dandan Technique
Arhat Demon-slaying Technique
Heroic Way

5. Xu Zhu

He inherited the cultivation of Wu Yazi and Tianshan Tonglao, over two centuries of combined cultivation.

6. A'qing

Ranked top in the 33 assassins in ancient China.

7. Zhou Botong

He learns martial arts for fun. He's mastered the Nine Yin Manual, Ambidextrous Technique, Seventy-two Kongmin Fists. All these on top of his years of cultivation as a Quanzhen first generation member.

8. Xiao Feng, Duan Yu

Xiao Feng mastered the full 28 Dragon-subduing Palms, placing him above Guo Jing in the rankings. Duan Yu learned series of skills and is immune to poison. Lingbo Footwork, North Sea Technique, and the Six Pulses Spirit Sword.

9. Guo Jing

Strongest of the New Five Greats. He mastered many skills

Demon Subduing Staff Skill
Bare Handed Disarming Skill
Bone and Joint Dislocation Hand
Golden Dragon Whip Skill
Southern Mountain Saber Skill
Mountain Splitting Palm Skill
Huyan Spear Skill
Yue Maiden Swordplay
Quanzhen cultivation
Nine Yin Manual
Eighteen Dragon-subduing Palms
Seventy-two Kongmin Fists
Ambidextrous Technique