Step 1 :The leading term of a polynomial function is the term with the highest degree.
Step 2 :In this case, the function is $f(x)=-x^{3}+9 x^{2}-20 x$.
Step 3 :The term with the highest degree in this function is $-x^{3}$.
Step 4 :So, the leading term of the function $f(x)=-x^{3}+9 x^{2}-20 x$ is \(\boxed{-x^{3}}\).