Hello. Can anyone recommend best roads (less traffic, more scenery) through Java and Sumatra? I leave by motorcycle on April 5th from Bali. Thank you.

Can't recommend roads, but I can recommend the Lake Toba area in Sumatra

https://flic.kr/p/7qbB5d https://flic.kr/p/7q7HRp
and Borobodur in Java:

https://flic.kr/p/7q7XYB https://flic.kr/p/7q7Ydk

We travelled down from Jakarta through Java and over to Bali by ferry mostly by train. Highlights included Mount Bromo, Mount Ijen, Yogyakarta/Borobodur, Magelan, the Dieng Plateau. All would be easy enough to string together on a bike ride but I would strongly recommend some off road experience and a decent bike and gear. Roads can be rough in parts, traffic awful and driving standards the worst I have seen outside of Africa. You are right to look for the roads less travelled, the main highways would be a nightmare on a bike. Some photos and detail of our time Java and Sulawesi on our blog @ https://accidentalnomads.com/category/indonesia/