For being able to take abuse and just keep going... My votes are DT360, DT466, 6BTA (5.9 Cummins)... ALL MECHANICAL. None of that electronic garbage... Bosch inline injection pump. A close 4th would be the 7.3 IDI but they are severely underpowered for a bus.
As long as they have oil, coolant, and fuel... they'll keep going. The 360 and 466 are built heavier so those would be my first choice.