When brushing your teeth, you should always aim to remove plaque and food debris which accumulates as you eat. Aside from that, a good toothbrush must also be easy to handle, and is gentle on the teeth and gums. A soft-bristled toothbrush, together with good brushing technique and habits, will be more than capable of getting the job done.
On the other hand, medium to hard-bristled toothbrushes tend to be too abrasive on your teeth, which can erode the gums and result in cavities not caused by bacteria. Over-brushing can also lead to increased tooth sensitivity, as the outer layers of enamel are stripped away and exposing the inner layer of dentine, which is more sensitive.