In JY's World, Tian Shan Tong Lao, Wuyazi, Li Qiushui, and Sweeper Monk are all very powerful fighters and probably the most powerful. Then again they are very old and have years of experience and Internal Energy. We know Feng Qing Yang and Dugu Qiu Bai are very powerful but we don't know how powerful they are because JY himself never expanded on their story.
In fact Dugu Qiu Bai never even makes an appearance in any of his stories, only mentioned in name. Wong Chong Yang and Lin Chao Ying are also very powerful as well, Wong Chong Yang defeated all the greats during the first Mount Hua tournament and obtained the Nine Yin manual, which Guo Jing and Zhou BoTong would later learn on Peach Blossom Island. And this manual would make an important impact later on during the years in ROCH and HSDS.
In Gulong's world the top powerful opponents would be Ye Nan Tian, Princess Yao Yue, and Princess Lian Xing. NONE of the TV adaptions show this properly though sadly. Chu Liu Xiang, Lu Xiao Feng, Xie Xiao Feng, Dagger Lee are all also top powerful fighters, but just a small level then the top three mentioned above.
In Liang Yu Sheng's world, there are so many outstanding fighters, and a lot of them are women (awesome and this is why I love his works). But some I can think of are Reverend Hui Ming (aka Yue Mingke), Yu Luo Cha (White Hair Demoness), Murong Chong (in the novel he is much more powerful then the TV series), Tie Feilong, Red Flower Demon, Yu Luo Cha's masters are also very powerful as well, they obtained sword skills from various sects and combine and change them all. (Huo Tiandu, and Ling Muhua, Huo Tiandu is Yue Mingke's master).
Zhang Dan Feng is also an outstanding fighter (he is Huo Tian Du's master, which makes him Yue Mingke and Yuo Luo Cha's grand master). Shang Guan Tin Ye and is also noted to be a very powerful fighter but he makes a guest appearance at the end.