My favorite waterfall in Nikko is easily Urami. The name translated means “see from behind”. There is a crude trail from the deck area that leads you to a spot directly behind the water falling. It is a bit dangerous and you definitely needs hiking boots to do it. There used to be a more developed trail from the other side but it was washed away in a flood.
Getting to Urami waterfall is hard. The side road is about 3km from the world heritage area and another 2.5km up that road. The best way to get there is using the 2 day bus pass that takes you to Lake Chuzenji, Senjogahara or Yumoto hot springs. The bus stop is on the main road.