Final answer:
The best estimate of glomerular filtration rate (GFR) is provided by the clearance of inulin, due to its consistent filtration properties in the kidneys without being reabsorbed or secreted.
Step-by-step explanation:
The substance that provides the best estimate of glomerular filtration rate (GFR) is inulin. Inulin is a plant polysaccharide that is neither reabsorbed nor secreted by the kidney after it filters through the glomerular membranes. Its concentration in the urine is directly proportional to the GFR. However, due to the practical challenges of using inulin in a clinical setting, creatinine clearance is often used as a more convenient measure, even though it is slightly secreted by the nephron along with being filtered by the glomerulus.