What you're asking for is an entire article's worth of info.
I use Essential Formulas Probiotics 12+ in my practice (blue box Professional Formula, not green box). It has always served me and my patients well.
Most of the time, especially if you are buying probiotics mainly based on price (the one I use isn't cheap), it is best to rotate the brand.
You want bang for your buck, get some kefir and take a swig every day or two. Rotate brands.