Yes another vote of the OH6, most will give you a good 500HP per ton (660 hp/ton for a light AS350).
I would think the winner lies into which aircraft can bring the thrust vector forward the fastest, specifically with proximity of the ground. that means low rotor inertia on the longitudinal axis (short blade and many them) and low fuselage inertia (short tail)
in the context of a dive you could get much better accel I am sure