On all matters, Chambers Dictionary rules supreme; on this we can all surely disagree. Chambers gives:
a top priority :
most important thing, most urgent matter, matter of highest/greatest importance, main thing, supreme matter, first concern, primary issue, essential, requirement, pole position
colloquial top of the tree
The OED, possibly even more authoritative, has separate entries for "top priority" and "low priority". For top priority it gives:
Of the greatest importance.
with the first quotation from 1941.