What pain medication is best for severe pain?

One that works. There is no best medication. You would be well advised to start at the lowest dose and titrate up under a dictor's direction.
Depends. The best medication depends on the source of your pain. For example, the best medication for migraine headache differs from the best medication for pain caused by a pinched nerve, or pain caused by a stomach ulcer. Start by seeing your doctor first to delineate the cause of your pain.