best do

(had) best (do something)

It is a good idea or in one's best interest to do something. I'd best get to bed early tonight, since I have to be at the airport at 3:30 AM. You best call your mother before she hears about your pregnancy from someone else.

(had) best do something

ought to do something, had better do something. Mary had best learn to mind her manners. You best listen to what I say.