A search of Answers.com revealed that "had best" has been with us since the mid 1400's yet it seems to be the poor cousin of "had better" and isn't used nearly as much. Answers.com defined it as "act in ones own or everybody's best interest." "Will do well is given as a synonym (I would have thought "would do well" is nearer, though.) "Had better" is given as a synonym of "should"/"ought to" (personally, I think "had better" is stronger than that and lies somewhere in meaning between "should"/"ought to" and "must"/"have to".
"Had best" seems a little anomolous in one way because "best" is the superlative form whereas "better" is only the comparative yet "had better" seems stronger. "Had good" as an idiom/modal does not seem to exist.
"Had best" also seems a little colloqual to me (perhaps thats why it seems less forceful.) That said, a google search of EFL "had best" produced some quite formal looking documents using "had best" in its idiomatic/modal sense.
A person's dialect seems to also determine use of "had best" - it seems far more common in London/Estuary English than in other dialects.
I wonder if you could think about the relationship odf these two phrases. Would you classify "had better" as the proximal form and "had best" as the remote for instance?
Well, I've got other things to do now so I'd best be going.
