The best triathlon hydration system is the one that meets your needs, not the one that looks the best! Fluid weighs a lot and you do not want to carry more fluid on the bike than you will need for your triathlon. A sprint triathlon may only require an bottle cage or torpedo bottle whereas an Ironman will need multiple sources. Torpedo systems or aerobottles fit between your arms and allow you to drink without leaving the aero position- which means more speed. Behind the seat systems are great for keeping additional fluid sources out of the aerodynamic slipstream, and can be used to "dump" into your front hydration system. Triathlon hydration systems now come with integrated computer mounts and storage spaces for nutrition, and certain systems can even reduce drag increasing performance. Check out the options from Xlab and Profile Design both leaders in this space.