Generally, "getting bugged" is something someone else does to you, rather than an emotion you feel like annoyance. For instance, we can say "I am feeling annoyed" but we wouldn't say I am feeling bugged". Most of these words gain their meaning contextually. For instance, if you said it bothers you when somebody leaves the dishes out, everybody would understand it's not as severe as something more serious that bothers you emotionally. I think that "bother" is the strongest word here, because it suggests a feeling less precise and perhaps overwhelming. Feelings like annoyance and frustration tend to pass quickly and be caused by a specific event, but that's not always the case with "bother".

I think "bugs" is milder, while the others are stronger, and equally so. All of these words can be used to refer to people, actions, or things, except "frustrate" is mainly used for things. So "you frustrate me" is ok, but less usual "Learning Chinese frustrates me" is more natural. However, you might get different answers from different people!

P.S. Probably It should be "the stress level" instead of "upset"

Hi, Peachey:) That's certainly true!! I meant native English speaker's best guessing. not "set up"

Just a note... "upset" means unhappy and in distress. "Set up" means organised or established. Did you mean "How are the percentages set up?" (This also means that someone did the work deliberately, and it wasn't by chance.)