vertices - Find the NW vertex of a rotated polygon

I am using QGIS 2.18 and I am wondering how to find the NW corner of a polygon and create an attribute of that point number. See the attached image. For example, please node 8 is the NW corner for Grid #1, 13 is the NW corner for Grid 5 and so on. any help with this would be wonderful. The two files I am working with is