What is the best toothbrush for remineralizing sensitive teeth?

Soft toothbrush. Actually, a toothbrush cannot remineralize your teeth. Certain toothpastes can be effective. Always use a soft toothbrush so you do not remove anymore of the enamel which will increase your sensitivity. Fluoride rinses are also very helpful to rebuild your enamel and decrease sensitivity.
Toothpaste? I think you mean toothpaste and not toothbrush since toothbrushes do not remineralize teeth. Most otc toothpastes don't have minerals in them but the Fluoride in toothpaste aids remineralization. If your teeth are sensitive try sensodyne pronamel toothpaste. It is a bit pricy but if it works for you then stick with it. You can buy a gel called ctx4 1100 from carifree.Com that has minerals in it.