Eggbutt is one of the most common snaffle bits and also one of the most gentle ones. It prevents rotation by keeping the bit stable. Many horses tolerate it better because it doesn't pinch their lips or sides of their mouth.
Loose-ring type of bit have a small ring (loosely attached) that allows the bit to move around in the horse's mouth. It encourages a relaxed jaw, but might require a bit guard as it's known for pinching the sides of the mouth.