Badly wilting browning basil
Look under the leaves for a pinkish discoloration/fuzziness. That would be downy mildew, which you cannot treat. In that case, use all the healthy foliage you can and throw away the plants.
Otherwise, the browning is probably from a root rot associated with overwatering. As weather cools, water evaporates more slowly and plants need less water as they are not growing as quickly, so watering routines need to adjust for that. Also, we've had a lot of rain lately and if your basil sat in water for a long time, some of the roots may have drowned.