Toughest duty? I don't think any duty in isolation is particularly fatiguing or tough. By far the worst of the worst is the constant grind of max duty, lots of short sectors, crappy forecasts, crappy ATC, crappy efficiency policies, early starts after RDOs with late finishes before. Then there's the passive aggressive sh!t from management when you decide not to extend or go fatigued.
It's the chronic nature of shabby rostering and predatory managers that are creating this fatigue (and morale) bubble. When it busrts is anyone's guess, but when it does, it's going to be fugly. CASA?? What are you doing?