When asked this question, I immediately think of painting tools. When I painted with oils I was introduced to red sable brushes and eventually discovered fake red sable (sabeline) brushes, and those are generally my favorites, though really, it just depends on what I’m trying to do.
I’m also a fan of disposable palette paper, because I don’t like cleaning my palette.
I also like palette knives, both for mixing painting and for adding texture to the painted surface.
What are your favorite tools for your work?