I think I've finally found an opponent who would've given Dook Goo Kau Bai a run for his money in a sword or sabre fight, and ironically, it's someone who shares his surname: Dook Goo 1 Hok from Gu Long's LUK SIU FUNG.
Why Dook Goo 1 Hok? Because according to Sai Mun Chui Sheut, Dook Goo 1 Hok had a unique skill: he could instantly identify the flaws in his own stance and seal them before the enemy could exploit them. That's why even Sai Mun Chui Sheut, who famously sought out and attacked the opponent's flaw, was unable to kill him before thirty moves. Even Sai Mun Chui Sheut had to admit that had Dook Goo 1 Hok's inner power not been drained (by Fok Teen Ching) before their duel, he probably would not have been able to kill Dook Goo 1 Hok and likely would have been killed by Dook Goo in return.
The young Dook Goo Kau Bai, who used Dook Goo 9 Swords (before his Heavy Iron Sword and No Sword stages), basically followed the same philosophy that Sai Mun Chui Sheut did (I've always believed that Dook Goo Kau Bai in his twenties was probably alot like Sai Mun Chui Sheut, and if Sai Mun Chui Sheut managed to live and improve into his 60s, he probably would've ended up like Dook Goo Kau Bai): identify and attack the enemy's flaw. If so, then Dook Goo 1 Hok might have given the young Dook Goo Kau Bai one hell of a challenge.
I don't know if Dook Goo 1 Hok would be able to handle Dook Goo Kau Bai in the latter's Iron Sword and later (i.e. Wooden Sword and No Sword) stages, however. By that time, Dook Goo Kau Bai had stopped attacking flaws and was just overwhelming opponents with sheer power. Luk Siu Fung said that Dook Goo 1 Hok had some seriously deep inner power himself, but it couldn't have been *that* good if Fok Teen Ching was able to drain him like he did. If Fok Teen Ching could drain Dook Goo 1 Hok's inner energy, then likely Dook Goo Kau Bai could as well.