What is the best highway tractor engine?

For which application, and what's your criteria of 'best'? Power, torque, fuel economy? Traditionally, the Detroit Diesel 60 series has been the leader for fuel economy. However, after 2002, they only became available on Daimler A.G. products (Freightliner, Sterling, Western Star). Caterpillar and Cummins generally have a wider torque band better suited to heavy haulage and sitework driving. However, Caterpillar left the highway diesel market in 2009.

You also have to consider weight. A 15.8 liter Caterpillar C16 is an attractive option for someone seeking power, but that's also a lot of weight, which may make it undesirable for some purposes - especially for someone who hauls by the ton.

Then, there's the application. If you're just doing regional runs in a flat area, then you can do fine with a 12.8 liter motor and 3.58 rears. But if you're hauling cross country in varying terrain, that setup may not be so ideal, and you may instead prefer a 14 - 14.6 liter motor.

So it's difficult to say which motor is the 'best' without knowing your criteria, but based on my own experiences, there is one I can tell you to avoid - the Volvo D16. A powerhouse when it runs properly - keeping it running properly is the difficult part.