At what altitude do winds exert the most force?
".....the density of air decreases with height, so the force of high winds aloft are lessened." I can personally vouch for the difference between a 60mph wind at sea level, and one at 14,000 feet! However, however, the single biggest difference between the two altitudes isn't so much the force of that wind, but the consistency of that wind speed, which seems to matter most to wind generation.

Here's how a wind turbine engineer would think of this problem:

From The Engineer's Toolbox- Power generated from the wind

Theoretically power in moving air - or wind - can be calculated

P = 1/2 ρ A v^3
  =  1/8 ρ π d^2 v^3     

where
P = power (W)

ρ = density of air (kg/m3)

A = wind mill area perpendicular to the wind (m2)

v = wind velocity (m/s)

π = 3.14....

d = wind mill diameter (m)