The 6.7 DOHC Cummins.
But gasoline? To be honest I think they all suck. All of these companies trying to capitalize on that 1 mpg, at the sacrifice of cost, reliability, and durability. BMW's really are decent if they are extremely well maintained and never modified ever. The EcoBoost has been a shitshow since the beginning, and the mpg is only good in EPA testing. The minute you put a human behind it and a GM pushrod V8 with double the displacement outperforms it in every category, including fuel mileage.
At this point in time, turbocharger technology or development is just not up to the rigors of gasoline engines, and GM has proven that.
The LSX is the best automotive engine ever designed.