Defining busy is not easy. If we agree on the number of a/c at a given time on the frequency divided by tthose the % of those on vertical or heading evolution, then some sectors at LATCC or Maastricht score very high indeed , but..
I worked in Maastricht with 25 + on the FQ and thought that was the max achievable, until I visited some US Towers ( Chicago, Atlanta or JFK on a friday afternnon was a mind opener I can tell you ) At least on those days ( 1990 īs) they beated us Europeans a long way.... They also got relieved every 20 minutes.