In my opinion is an F award the best use for Ey miles. I give you an example why. I usually fly FRA-AUH and return. Y ticket cost usually about 500€ if you would like to upgrade both sectors it would cost about 54,000miles. An C award cost usually about 64,000miles plus fees. The last F award I got was about68,000miles plus Fees (I paid an extra 150€) and used the miles plus cash option.
Please not that I do not remember the exact amount of miles. It might be possible that I spent a little bit more