The best test for GFR is with
Properties of Inulin
-        Easily filtered by kidney
-        Not absorbed by kidney
-        Not secreted by kidney
-        Not metabolized by kidney
-        Not synthesized by kidney
-        Non-toxic
-        Easily measurable
Thus it is the best substance to use for measurement of GFR.
Inulin
Hippuric acid
Creatinine
PAH