MUST
очень настойчивая рекомендация, настоятельный совет (ну, просто должен):

You must read Sebastian Faulks’s latest book. It’s stunning!
You must do something about that cough.  Please go and see  the doctor.
He must get the driving license if he is after this job.

При этом, на must делается ударение.

SHOULD / OUGHT TO (особой разницы нет)
следовало бы…, лучше бы…

This soup is too salty!’ ~You should / ought to send it back.
You’ll catch cold if you go out like that. I think you should / ought to take a hat.
Why are you sitting here doing nothing? You should / ought to be working.
НО
should (would), а не ought to:
– c ”I (я), we (мы)” – я бы…, мы бы…
I should leave early tomorrow, if I were you. (ИЛИ I would leave…; ИЛИ I’d leave…на твоём месте, я бы…)
If I were you, I wouldn’t go to that meeting alone.
Look outside. In weather like this, I would stay at home.

– предпочтительнее в вопросах (спрашиваем совет)
What should I do if I have any problems?
Should I ring you at home?
Where should I put this picture, do you think?
It’s a difficult problem. How ought we to deal with it?

HAD BETTER – вместо should ИЛИ ought to (особенно, в разговорном английском; и оно выразительней)

If you’re not well, you’d better ask Ann to go instead (… лучше попроси Анн пойти вместо тебя)
There’s someone moving about downstairs. We’d better call the police, quickly (… нам лучше вызвать полицию)
He’d better not be late again, or he’ll be in trouble (Ему лучше не опаздывать…)
Had we better get a taxi? ИЛИ Should we get…? (Может, нам лучше взять такси?)
НО
не используем:
– в Past
You should / ought to have caught a later train, (NOT You’d better have caught…)

– в общих (не конкретных) случаях
I don’t think parents should / ought to give their children sweets, (NOT …parents had better give…)