Probably Katsura. Shoukaku was stated to be the strongest of the Harusame, which should probably include Kamui, yet Katsura was still able to beat him in a fist fight, whereas Sougo's short bout with Kamui was too short to judge them as complete equals.
Well Kamui seemed to slightly get the better of Gintoki, even before he went all Yato rage. If not for Kagura and Abuto's intervention he would have killed Gintoki. So maybe whoever said that wasn't aware of Kamui's power. Or was Kamui even still considered part of the Harusame at that point?
Zura fist fights Shoukaku in the manga? In the anime he fought with a sword of some sort mostly and then beat him with a headbutt.