Assuming that Dook Goo Kau Bai really was all that he was said to be, he didn't meet too many martial artists who impressed him during his own lifetime. Who throughout wulin history before or after his time might have impressed him, however?
We know that Yam Ngor Hang (SOD) would have impressed him; Jin Yong explicitly claimed that Dook Goo Kau Bai would been pleased to encounter an opponent such as Yam.
Yeung Gor would probably have impressed Dook Goo Kau Bai since he managed to master Dook Goo's own Heavy Iron Sword Technique at a very young age, and came very close to mastering the Wooden Sword Technique by the end of ROCH.
Fung Ching Yeung and Ling Wu Chung naturally would have impressed Dook Goo Kau Bai since both managed to master the principles of Dook Goo 9 Swords.
Who else would have impressed Dook Goo Kau Bai?