Kokaino is right on the money, as he always has been. IME, nitrazepam is by FAR the best benzo when it comes to muscle relaxation, if you want you muscles to turn to jelly, nitrazepam's your best bet. I find that it even eclipses diazepam to a very noticeable extent. Nitrazepam is personally my all-time favorite benzo due to its sheer excellence in all the attributes that would make a benzo enjoyable (muscle relaxation, sedation, anxolytic effects, mood boost, etc). Also in my experience, bromazepam is actually pretty good for muscle relaxation and is overall a pretty damn good benzo (not quite as good as nitrazepam and diazepam though). Oh and for the record, I've always found a strange thing about temazepam. On its own, it's pretty good, but when combined with oxazepam, it's outstanding. I never understood why this is the case as oxazepam is so mediocre on its own.
As Kokaino said, tetrazepam has extremely strong muscle relaxant qualities, so if you can find it, give it a try. I recall a regular on these boards from years ago that was extremely fond of tetrazepam, saying that despite its low potency, it was actually really euphoric and enjoyable stuff. I can't remember the poster's screen-name, except for the fact that he was from France....