If you are creating this by adding the fields one by one, just have a 'highest' index to keep track at the point you're adding them.

Code:
var %item = myitem
var %data = 10
hadd table %item %data
if (%data > $hget(table,highest)) hadd table highest %item

var %highest = $hget(table,highest)
echo -ag Highest item is %highest, data is $hget(table,%highest)


If you need to loop though an existing table:
Code:
var %i = 1, %n = $hget(table,0).data, %high.item, %high.data
while (%i <= %n) {
  if ($hget(table,%i).data > %high.item) {
    %high.item = $hget(table,%i).item
    %high.data = $v1
  }
  inc %i
}