The answer is Ambulance. Fire engine and fire truck probably "splits the vote" ...
AMBULANCE ranks #1,
FIRE TRUCK ranks #2,
and FIRE ENGINE ranks #3.
There's actually a difference between a fire engine and a fire truck, but to most people a fire engine is probably considered the same as a fire engine. However, in the fire service, the bigger fire truck has the larger main aerial ladder. The fire engine's primary function is pumping water, but it includes portable "Ground Ladders."
