What supplements actually work?

Evidence-based (strong data): Creatine monohydrate (5g/day), Vitamin D (if deficient, 2000-4000 IU), Omega-3/Fish oil (1-2g EPA+DHA), Magnesium (200-400mg glycinate). Moderate evidence: Ashwagandha, Zinc. Mostly marketing: BCAAs (just eat protein), fat burners, testosterone boosters, most greens powders.