^^Agreed, the A10 makes a sound similar to what I imagine the World coming to an abrupt end would sound like.
I'm surprised Nobody mentioned it, but My personal favorite is the SR-71 Blackbird. What an absolutely remarkable piece of engineering.
And although the thing never got fielded, I was always fascinated by the RAH 66 Comanche Stealth heli
As for rotary wings that actually exist
, I'll pick the AH 1W Super Cobra