What is the most common muscle disorder?

Sprains. Sprains/strains is the most common muscle problem. If you are looking specifically for a disease, then muscular dystrophy would be my top choice. Depending on how you want to classify fibromyalgia, it could rank as the top disease.
Low back pain. In my practice, low back pain and neck pain of musculoskeletal origin is the most common. Shoulder rotator cuff issues are also very common.