A reader from Australia emailed me the following:
You may be interested to see what the Courts of New South Wales make of the “difference” between “best endeavours” and “all reasonable endeavours”—pretty much nothing (usually), which reflects what the law in Australia has been for over 20 years.
Here’s how a recent Chief Justice of our High Court put it: “[A]n obligation to use best endeavours does not require the person who undertakes the obligation to go beyond the bounds of reason; he is required to do all he reasonably can in the circumstances to achieve the contractual object, but no more.”
That being said, in the recent case of Waters Lane and Anor v Sweeney and Ors  NSWCA 200, the trial judge did suggest that there may be a distinction in some contexts along the following lines: “best endeavours” may take into account the capacity and ability of the party making the commitment whereas “all reasonable endeavours” is not concerned with the party’s capacity—just with what is objectively reasonable.
I suggest that in the Waters Lane case the judge was just doing what misguided judges are wont to do—making stuff up. In terms of the semantics, his or her purported distinction is obviously up the creek. (See here if you’re not convinced.) It’s also outlandish to propose that contract parties and their counsel could have such a rarefied distinction in mind.
As usual, the goal should be to avoid at all cost any squabble over the meaning of an endeavours (or efforts) provision. The way to accomplish that would be to stick with reasonable endeavours (or reasonable efforts), providing a definition in sensitive contexts.