For me it's gum (especially completely open mouth chewing... like, seriously?!) when working in close quarters like conference rooms. I don't say anything, because I don't want to seem rude or controlling. Chewing coming from nearby cubicles is also very noticeable to me. When I hear it, I have to escape with headphones. I try to keep in mind that for the person making the noise, it is a positive thing in their perspective (as it is for me when I eat), and not think "should" statements.
As a person who hates the sound of people chewing loudly, when you eat at your desk, please be considerate of people like me. Chewing with your mouth open bothers more people than you think. Should fall under standard don't-bother-people decorum like showering and brushing your teeth but it doesn't.