OBLIGATION
be a must
to be something that you must do. (Informal.)When you're in San Francisco, see the Golden Gate Bridge. It's a must.It's a must that you brush your teeth after every meal.
had best do something
ought to do something.You had best get that fixed right away.You had best be at school on time every day.
had better do something
ought to do something (or face the consequences).I had better get home for dinner, or I'll get yelled at.You had better do your homework right now.
if one knows what's good for one
phr. “one had better do what is expected of one.”You'd better be on time if you know what's good for you.If you know what's good for you, you'll call and apologize.
(must) do and must
n. something that someone ought to do. (Always with a.)Seeing the Eiffel Tower is a must do in Paris.This is a do when you're in town.