Just speaking for my progression....

Hovering at altitude
The Wall to a Hover
Waterfall to Hover
Blender to Inverted Flat Spins
Blender to Upright Flat Spins
Parachute to Inverted Elevator
Parachute to Elevator
Inverted Harrier
Harrier
Harrier Landing
Torque Roll
Tail Touch
Rolling Harriers

Doing all the above at a very low altitude becomes the biggest challenge...Personally I think a Rolling Harrier inches off the deck is the most difficult and has the biggest wow factor of all the maneuvers.