As in chewing or crunching on something, specifically when something is chewing loudly or roughly, like an animal eating.
When chewing soft food loudly with mouth a bit open, I use くちゃくちゃ. It may become くっちゃくっちゃ or ぐちゃぐちゃ, depending on person and area. I guess, many Japanese people hate to hear the sound from such a way of eating, and this word may contain such attitude hating the chewing sound.
When chewing hard food (like rice cracker) and making sound, I use ぼりぼり. If the food is so much hard (like ice cubes or fish bones), then I may choose がりがり or ばりばり.
If you want to express the attitute of eating, then もりもり gives positive and enthusiastics impression. For example, I chose it when kids are eating well for good health.
On the other hand, がつがつ gives negative impression and it may represent eating habit with bad manners. I also use it when eating with so much hunger, but I don't mean much negative sense in this case.
The choice of the words would differ depending on how you feel about the habit or the behaviour.