The key is not what the pay is but where you spend it!
I agree the highest numbers are probably in the UK N. Sea, though I seem to recall the Norwegians were on fairly high salaries too, but the actual quality of life/standard of living may well be lower than others on lower salaries elswhere.
I'm sure a Canadian or American offshore captain on 60-80,000 dollars enjoys a much higher standard of living than his UK counterpart on 60-80,000 pounds. Same for the Australians. Easy to refute, but those who have seen both sides will generally acknowledge it to be true.
I would guess the highest paid in real terms are those working for the large international operators, on Sterling or US dollar salaries overseas, tax free, but living in countries with low costs of living e.g. S. Aficans, Australians etc. Certainly from what I observe around me, they seem to be doing pretty well!