Most profitable and easy are not the same thing.
Most profitable SNG format long term is no doubt 6 max hypers.
You can play hundreds if not thousands of games per day and limits that they ran on are highest on Stars.
Downside of course is the variance and thinner edges in that format. So anything lesser then SNE chase is not worth to pursue.
If you want something easy, 9 max vs 6 max is much easier to master. Tighter opening and defending ranges pre flop; less post flop play overall = less troubling spots, easier for multi tabling ; ICM based mostly (master ICM and you can crush it).
I would also add that despite 6 max hypers being profitable long run, believe that you can also make even more money if you played HU hypers from 60s+. I know for a fact that guys that play 30s can make all the from 3 to 7k per month. Assume guys grinding heavy 60s would make 6 to 10k etc.
If it was for the profit, believe it is easier to achieve it via 6 max hypers. If you want to learn better pre flop and post flop lines + exploit opponents long term, HU is better choice.
I do 6 max turbos, 15s and 30s, this year with average ROI of 3-4%. Have low ITM on 30s (less then 36%!!!) but you can have 4-5% ROI at 6 max turbo midstakes. Also next jump to 60s, mixing 30s and 60s 6 max turbos , 100 games a day, 300 days a year (2 months rest a year) , you can make same money as SNE makes BE with 3% avg ROI which is much more realistic then hoping for BE on 6 max super reg infested hypers.
Best of luck with your decision and great thread!