There isn't one.
Let me explain...
A guy called Dr. Simeons found that overweight individuals could lose weight extremely fast when they received a small dose of the hcg hormone. This is true - trials did show weight loss in obese individuals although a proper double blind study was not carried out so there are questions around exactly what did cause the weight loss.
So let's assume that HCG (Human chorionic gonadotropin) does actually help with fat loss, there is still a problem. HCG is a glycoprotein hormone, a very complex amino acid chain. Such hormones are actually quite delicate and as such cannot be taken orally as our stomach acids and digestive enzymes effectively break them down.
So the only real way of getting any HCG into your system is to inject it.
HCG Drops are oral, they won't work. Web sites and people claiming they do are relying on the fact that most people when they start using such bunk rubbish actually clean up their diet at the same time, so of course they lose weight.