What semi tractor truck gets the best MPG?

The MPG a truck gets is going to depend on a number of factors, such as the drivetrain, weight, terrain, etc. Another important factor which affects the mileage a vehicle gets is how its driven, as well as the type of trailer you're pulling - a regular van trailer can be made quite aerodynamic in conjunction with a tractor, but trailers such as flatbeds and their loads typically don't create ideal aerodynamics.

AFAIK, no manufacturer claims their truck to be the most fuel efficient on the road, although they do place emphasis on features which can make a truck more fuel efficient, such as lighter weight, aerodynamic design, etc.