Piper L-4 Grasshopper
The Grasshopper was the military version of the J-3 Cub (although the only real difference between the Grasshopper and the Cub was a coat of olive drab paint) an airplane famous for for helping hundreds of thousands of pilots earn their wings. It was slower than molasses going uphill in January, but could land or takeoff almost anywhere, was easy to fly and cheap to own. The Cub remains a popular plane among civilian pilots today, and hundreds of them are still flying.
Cool stuff for L-4 lovers:
The liason pilots don't always get the credit they deserve, but here's one first person narrative of what it was like to be Alone and Unarmed in the war torn skies of World War Two.