Sai Mun Chui Sheut fought many battles in the LUK SIU FUNG novels, but his two most dangerous opponents were undoubtedly Ngor Mei Sect Leader Dook Goo 1 Hok and the Lord of White Cloud City, Yip Goo Sing. He came close to dying by the swords of each of these two opponents.
Throughout the LUK SIU FUNG saga, there were other martial artists who could have posed a threat to Sai Mun Chui Sheut: the Wooden Taoist/Old Sabre Tyrant, Gung 9, and Luk Siu Fung himself, among others, but Sai Mun Chui Sheut never fought against any of them. Dook Goo 1 Hok and Yip Goo Sing were the toughest opponents Sai Mun Chui Sheut actually went up against.
What must have been frustrating for Sai Mun Chui Sheut, however, is that both of these victories, considered his greatest, were adulterated by extenuating circumstances that put his opponents at a disadvantage. Dook Goo 1 Hok fought Sai Mun Chui Sheut without the former's great inner power because it had been drained in an earlier fight against Fok Teen Ching, allowing Sai Mun Chui Sheut to win a fight that even Luk Siu Fung didn't think his friend could win. Similarly, Yip Goo Sing had Sai Mun Chui Sheut on the verge of defeat, but gave up and let himself die by Sai Mun Chui Sheut's sword rather than be executed by the imperial government for his botched assassination attempt on the Ming emperor. For the rest of his life, Sai Mun Chui Sheut had to wonder if he could have won either of those fights without those special circumstances, and of course, wulin would always be talking behind his back about how he was "lucky."
In any case, was Dook Goo 1 Hok or Yip Goo Sing the more dangerous opponent for Sai Mun Chui Sheut?