ORIGINAL: bogbeagle
Well, there's only 4 aerobatic manoeuvres, that I know of.
Roll.
Loop.
Stall turn.
Spin.
Everything else is a variant of these.
I have no idea what might be considered the "most difficult manoeuvre". There's plenty that I can't do with finesse. I did once see a guy do a rolling loop, if that helps. And, it wasn't a whizzy-whizzy blur of activity, either ... it was slow and measured.
Yeah, this is the thing, the rolling loop is similar to the rolling circle, it 'can' be completed by carefull well timed elevator only inputs. For me the holy grail of manoeuvers is the one roll rolling circle. Ever since i watched Hanno prettner perform it at sandown park in the 1980's i have been obsessed, but to no avail so far.
The difficulty lies in the fact that all four controls have to be varied constantly from start till finish at around 8 seconds. If it's flat calm you might get off lightly with the throttle.