I would like a recommendation of the bus company from Hanoi to Nanning please.

You can take bue from the Red River Hotel, at, 204 Tran Quang Khai St. Hanoi. It costs around CNY150 and takes about 7-8 hours. You need to change a bus after going through the border.
Well, there is also direct trains between the two cities. You may choose it which is more comfortable than bus.