Which peak on Mt. Huashan should we start off if we want to hike plank road?

Plank road is located at the middle of South Peak, which is the highest peak of the mountain. You can take a cable car to the North Peak. Then walk to the East Peak and further to the plank road. This is just for your reference.

Let me know if you have further confusion.

Personally, I found this the least tiring way - going up via the West Peak cable car, climbing up West Peak, then head to South Peak, the Plank Walk is close to the South Peak. From there, it is easy to descend and then to ascend East Peak where you can try the Flipping Sparrow pass, then head to the easy Central Peak, before making the long descend to North Peak via Black Dragon Ridge and Golden Lock Gate. From North Peak, take the cable car and then Bus back to the Visitor's Centre. It was much easier to follow gravity than to fight it!