I don't remember which specifically, but I searched and was really impressed with PB 8 at vitacost.com. check it out, and you can see the winners on there. variety is important. L. rhamnosus and L. Reuteri (not in PB 8) were two that i read about recently, very important for preventing gut infections of salmonella, rotavirus, and e. coli.
the gut is colonized at birth and different populations increase/decrease based on what you eat. obviously it's more complex than that, but there's the start. some studies are showing that specific bacteria make it through the stomach and colonize the upper intestine without special coating on the pills.