Horse bits come in a huge variety of styles and each offers a different type of control. It is usually best to start out with a light bit and then see how well your horse responds. If your horse can follow your cues with a light bit, then that bit is strong enough for now. If your horse doesn't respond or notice the bit, then you need a little stronger bit. One of the lightest bits are called Snaffle Bits and they are used by horse trainers who use the technique of controlling a horse through kindness and the least amount of pressure, rather than using a heavy bit that may hurt the horse.

Snaffle Bits have three common types:
D-ring bits are very light and are the best to start training your horse with as they have rings that are placed farthest away from the horse's mouth.
The eggbutt snaffle bit has rings that are affixed in one place and they do not move. These keep the rings from rubbing and cutting into the horse's mouth.
The loose-ring snaffle bit has rings that are free moving and sliding which keep the horse from grabbing the bit, as many spirited horses will try to do. When the horse attempts to grab this type of snaffle bit- it simply rotates away.

If you have a horse who has a tendency to munch on his bit, resign yourself to the fact all you can do is reposition the bit time and time again until you get to the root of what is causing the bit munching. In many cases the horse was trained early on using a bit that was too heavy and the horse is anticipating being hurt by the bit. Horses are highly intelligent and once something goes awry in training, they tend to remember negative experiences with training objects as well as positive ones. Continue gentle training and if your horse continues this behavior, also have his teeth checked.

If you can rule out the horse's teeth bothering him, start using a light bit every time with your horse and continue gentle training techniques and he will understand the bit will not hurt him and it will cut down and eventually end the bit munching.