There is no best painkiller. The treatment for pain is according to a step therapy, which means that when the pain is relatively mild, some conventional painkillers are used, such as non-steroidal anti-inflammatory drugs, such as ibuprofen and celecoxib. If the pain is more severe, you need to use drugs with relatively stronger analgesic effect, such as tramadol, diazoxide, etc. If the pain is caused by tumor or is very severe, you need to use opioid analgesics, such as pethidine (also known as dulcolax), morphine, fentanyl, etc.. Because these drugs are very strong analgesic and addictive, and can cause respiratory depression and so on. If not used properly, they can lead to addiction and cause respiratory depression and death.