Recently went to the AMERICAN ARMOURED FOUNDATION of Danville, Va. They have several rare AFV's M4A3E8 Sherman
with an M2 plow and a Pk IVG are some of the rarest. They also have a M20 and Dingo in their collection. These last 2 AFV's are some of the smallest AFV's I have seen.
The rarest modern AFV I say there was a M-108
http://www.aaftankmuseum.com/
http://www.aaftankmuseum.com/Tank_Ordnance.htm