What is the strongest muscle relaxer?

Not one agent: There are many relaxants. I believe one medicine may work better than other in a given individual and it will also depend what is the underlying medical problem being treated.

All will work: Anesthesiologists would probably say pancuronium or the like, but i doubt you are under anesthesia. For muscle spasms, just about all will work at their particular effective doses. Multiple classes so like apples and oranges. Your doc will find the one with most-tolerable side-effects. Remember, exercise and stretching probably work best long-term.