The best answer to “Which probiotic is best?” is: “Best for what goal?” Probiotic effects are strain-specific—different strains within the same species can produce different outcomes. Use a decision framework: (1) Is the product supported by human studies for your target outcome? (2) Does the label clearly list the species + strain and the CFU dose? (3) Is viability/stability supported through the end of shelf life? (4) Does the manufacturer provide quality documentation or testing? For gut comfort, some Lactobacillus and Bifidobacterium strains are commonly studied; for antibiotic-associated diarrhea, different strains may be more appropriate; and for women’s/urogenital health, other strains are typically considered. Probiotics are not magic on their own—fiber intake, hydration, sleep, and stress management often amplify results. In high-risk medical settings, probiotic use should be clinician-guided. If no clear benefit is seen after 2–4 weeks, consider adjusting strain/product or switching strategy (prebiotic/synbiotic/postbiotic) based on the goal and tolerance.