The probiotic strains that have been proven most beneficial in the medical journals are Bifidobacterium infantis (B. infantis), Lactobacillus acidophilus (L. acidophilus), and a couple other Bifidobacterial species. My GI and I both swear by VSL#3, which is found at some pharmacies. It has literally changed my life, cutting the frequency my bathroom trips in half (from >10 times a day to about 5-6). Prior to VSL#3 I had tried Culturelle, Nature's Way, etc. but this one certainly works the best for me.
Each person is different so it might take a few experiments. The bottle will tell you if refrigeration is required; it's not a bad idea because it extends the shelf life of the live cultures.