Buprenorphine is a tablet that melts under the tongue- I've had it and Pethedine a few times. Pethedine is better at pain relief than temgesic/ buprenorphine in my experience. Pethedine is fab. I'd happily live on that all day when in pain, but it is super strong so only to be used when pain levels are 8 or 9/10 and above.
The temgesic makes me nauseous and throw up, but the Pethedine didn't have the same icky side effect for me. Pethedine therefore gets my vote as an amazing pain killer.
Entonox is only any good while you are inhaling it, the effects do not last long.
Tramadol will last longer, and is very good when given in to the leg or bum muscles for speedy relief.
The difficulty with all the ones you take orally is when the pain is too much you tend to vomit and up comes the remains of any tablets. The only way to bypass that risk is to have injections of pain relief at a hosp. Or do as I did in the past, get a GP to prescribe ampules of Pethedine, you keep them stored at home securely, and when the need arises, and you have to call out the GP or paramedic in the middle of the night, they can bring the syringe to administer it to you. It is not something you should be self injecting, and call out Docs do not carry that strength of meds with them nowadays for their own safety as they are class A drugs. But if you have them at home already, they can give the dose to you that you require.
If my experience is anything to judge, don't dismiss the Pethedine, it really is what is needed for excrutiating pain at times. It's like morphine. I believe you can get tablets of the stuff too, but when the risk of vomitting is high, then the injectable sort is best.
My endo pains are currently okay with diclofenac though it does make me even dopier and drowsier than normal. It's a muscle relaxant pain killer and that seems to help me a lot.
Entonox is just what paramedics give to calm you down when you are at risk of going in to shock with pain. Not really a solution for severe endo/gynae pain.