Tough question to answer: the binding capacities of different opioids at the mu receptor vary depending on the assay method chosen and tissue type. Also, the binding capacity somewhat depends on what other drugs are present in the environment. Separate from binding capacity is intrinsic activity, and while often considered the same, they are not necessarily closely linked. The partial agonist buprenorphine has a relatively moderate to high binding capacity and a modest intrinsic activity whereas sufentanil has a much higher binding capacity and intrinsic activity. The half life of buprenorphine is long (36 hours) that is much longer than many of the opioids, so ostensibly the pharmacokinetics preserve relatively high levels at the receptor and may be responsible for at least part of the effectiveness of the drug to block others. There are several opioids with higher binding capacities than buprenorphine including hydromorphone, sufentanil, and fentanyl.