First off, there is no "best" probiotic. The very best thing for your guts is an extremely diverse set of commensal bacteria that have adapted to cooperate with

First off, there is no "best" probiotic. The very best thing for your guts is an extremely diverse set of commensal bacteria that have adapted to cooperate with the human body, preferably inherited from your family down many generations. Hardly anyone has that any more, thanks to generations of antibiotics and diet lacking a variety of fiber. Probiotics are like a mercenary army that you can bring in when some pathogens are causing havoc or the equilibrium is very disturbed. They generally don't stick around permanently but they help outcompete the nasty microbes. Unfortunately, they also compete with the good microbes in many cases (the "standing army"). We really don't have a good process to restore the species that get lost through antibiotics and bad diet other than good diet for a long period of time (a wide and balanced variety of whole plant fiber) hoping the good bacteria will establish themselves again, or a fecal transplant which has all the risks of a blood transfusion if the donor isn't screened thoroughly.