Q: What is the best insulation for walls? Fiberglass vs Cellulose vs Rockwool vs anything else

A: If by best you mean the highest R value per inch then the rank order is SpaceLoft (R10/inch), spray foam or ISO boards (R5 per inch), dense-packed cellulose and Rockwool (R4 per inch) and fiberglass (R3 per inch). However, fiberglass is the cheapest per R value and you can install it yourself.