Nothing out there is truly bad.
OK, you need to try an early Saf-T-Liner with a 504 or a 555 Cummins on for size!
Wait, a triple-nickel Cummins needs a
Seriously: for mountains and towing, I'd go DT466, Cummins 8.3, Cat 3126, or Ford/New Holland 7.8 litre. Not so much for the engine, but because gassers and the smaller diesels tend to have the light-duty AT545 transmission behind them. The larger engines will have the much-stronger lockup MT643 Allison or, on newer buses, the computer-controlled MD3060. Some AmTran pushers have an MD3060 behind an International T444E...good driveline, but mountains with a 5000+lb Bronco on the hitch might be pushing it for the hillclimbing power.