“What medicine works best for IBS?”

Well, IBS is a somewhat complicated diagnosis because every patient seems to suffer from different symptoms so treatment can be quite variable. The most common medicine that people usually use is an anti-muscle spasm medicine called dicyclomine. This mainly helps with the pain that comes along with IBS. Several other treatment options have been studied and include diet therapy as well as some over-the-counter supplements such as IBgard.

Depends on the type of IBS. Diarrhea or constipation predominant? Many therapies are available (over the counter and prescription) and they need to be tailored to your specific needs. Here is an overview on treatments:
https://www.mayoclinic.org/diseases-conditions/irritable-bowel-syndrome/diagnosis-treatment/drc-20360064

You will need to visit a GI specialist before being prescribed any meds. Remember that your diagnosis must be confirmed by the physician who treats you. Treatment is not available on this site.