Instead of going for a probiotic, where more than half of the microbes exit the factory dead, go for some home-made goat kefir (must be home-made for the advertised potency). Way more microbes/yeasts in there (5 trillion per cup, instead of a few dead billion in a capsule), and they are the kind that actually colonize the human gut (yogurt and probiotic pills don't).
i alternate between renew life probiotics and VSL. renew life is better quality than VSL since it is pure probiotics- no fillers and sweeteners. VSL uses cornstarch, silicon dioxide, and maltose in their products.
however, VSL has a higher amount of probiotics pure sachet. i just finished a box of the VSL DS which is 900 billion per sachet. i don't think you can beat that amount.
I take Garden of Life Raw probiotic. It was the highest potency multi-strain probiotic Whole Foods had to offer, though I'm not sure where it ranks compared to other brands. I had some digestive issues that paleo never cleared up, (I suspect leaky gut, though I was never diagnosed.) and after about a week of taking the probiotic my digestion was much better. Also helped me with my dairy intolerance. (hooray!)
As to whether it actually colonized my gut or not - I had to be on antibiotics 2 weeks ago for an inner ear infection that wasn't going away on it's own, and never once had tummy troubles while on it. (I didn't take the probiotic during my antibiotic course) I think having somewhat stabilized my digestive bacteria beforehand definitely helped. I am back on the probiotics now that my antibiotic course is finished, and I feel just as good as if I never had to take any antibiotics.