Creatine monohydrate is probably the most effective supplement on the market for athletes and bodybuilders. The monohydrate form has the most research and evidence behind its efficacy and is the one I most recommend. A loading protocol doesn’t appear to be necessary, so taking 5-grams a day should do the trick.
The Study: Kreider, R. B., Kalman, D. S., Antonio, J., Ziegenfuss, T. N., Wildman, R., Collins, R., … & Lopez, H. L. (2017). International Society of Sports Nutrition position stand: safety and efficacy of creatine supplementation in exercise, sport, and medicine. Journal of the International Society of Sports Nutrition, 14(1), 18.