What is the gentlest horse bit?

Could you please explain to me in detail what is considered to be the gentlest horse bit? I'm looking for a bit that is comfortable for the horse and easy on their mouth, yet still provides enough control for the rider. Are there any specific types of bits that are known for their gentleness, and if so, what makes them stand out as being the most gentle? I'm interested in learning more about the design features and materials used in these bits, as well as any potential drawbacks or considerations that I should be aware of when selecting one.