General discussion about any of the popular chassis and cabs used by cutaway transit buses, shuttle buses, or motorcoaches: Ford E-Series, Ford Transit, Ford Super Duty, Chevy Express, Freightliner etc.
I'll start off with the 2017 Ford Super Duty. For 2016 and before, the Super Duty cab chassis variants (including buses) have been stuck at 300hp and 660lb*ft for the 6.7 diesel, while the pickup version's torque and horsepower have continued to increase. However, for 2017, the all new Super Duty will be revealed, and Ford claims upgraded powertrains for both the pickup and chassis cab. Ford's Super Duty is very popular with cutaways; for example, the Eldorado Aero Elite. The Super Duty chassis already makes for a very powerful bus, so it would be exciting seeing an even more powerful chassis.