What pain medication is strongest for chronic back pain?

No good answer. There is no one pain medication that works the best for back pain. Every medicine works to different degrees in each patient. It's more important to identify the cause of the back pain and target treatments at fixing the problem rather than masking the pain with meds which will typically have a short duration of efficacy. The cause of the pain will also dictate the class of med needed.
Complicated Answer. It is not so much which medication is strongest, but rather which one is best for you depending on the cause of your pain and anticipated duration. Also, one must consider kidney and liver function when prescribing various types of pain medications. Some of the classes of drugs used to treat pain include narcotics, anti-inflammatories, anti-depressants, and topical applications.