As a general rule, use a medium or medium soft brush with a small head. Hard filament brushes are less flexible and do a poor job at cleaning. However, recent evidence shows that electric brushes are more efficient than hand brushes.

Remember, however good the brush, it all depends on the person using it! You also need to use some other cleaning method, e.g. floss or interdental brushes to clean between the teeth. Ask your dentist or hygienist for advice.