There is no one best probiotic. We all are a bit different and what your body reacts well to and where your deficencies are are going to be unique for each person. I have tried primal defense, PB8, Yarrow, pearls, and others with different responses to each. With so much talk about primal defense, I decided to give it a shot, but am not feeling that it is worth the money. I had the best response by taking a combination of yarrow and pearls: 3 pearls at night, 5 yarrow in the morning.
You also need to consider kefir - either milk or young coconut. These are great sources for inexpensive, huge amounts of probiotics. I use young coconut kefir - 4 oz a day. I have to say, I actually rather like the stuff too.
Read all you can, but then do a bit of trial and error on yourself. Only you can find the right answer for you.