If you were teaching a group of learners, each of whom had different motivations for learning English, which learners would be the most challenging in terms of motivation?
I’m not sure there is an easy answer to this. The obvious answer is the student that is just there because their parents or employer have sent them. However parents/employers can be very strong motivators, especially if there is some kind of reward or punishment associated with rate of progress.
Another group might be the casual learner. For example, English teachers going to Korea often start off wanting to learn some Korean, but when they realise that they don’t actually need to learn anything, that they can get by with just English, hand gestures, Google Translate and pointing at things, they often give up soon.