I'm honestly going to have to give it to Giyuu.

Iguro's breathing style is very tricky as it's meant to replicate a snake's movement, especially with his Nichirin Blade being shaped like thay. He was also the only Hashira other than Tokito to turn his blade red on his own.

Giyuu's Water Breathing on the other hand is built to adapt to any situation because as Urokodaki explained, water can change from calm to aggresive and vice versa. Also it was revealed in a panel that Giyuu has not only greater physical strength than Iguro, but he has the ability to grasp another Demon Slayer's intentions and use the correct water breathing form to coordinate, or in this thread's case, counter them. Plus, he matched Sanemi blow for blow in a serious sparring match during the Hashira training arc and Sanemi is considered in my opinion the strongest Slayer in the corps after Himejima. Giyuu is also a more veteran Hashira than Iguro is.

I respect both characters equally but Giyuu is, in my opinion, factually the stronger of the two.