What is the least dense layer of the sun?
That would be the corona--the outermost layer, and, to use a terrestrial (and not really equivalent) term, its atmosphere. The corona is the bright halo of light visible only during a total eclipse. Its density at its base (closest the Sun's surface) is only about 400 million atoms per cubic centimetre. This may seem a large number, but the earth's atmosphere is about 10 trillion times denser at sea level. Moreover, the corona's density drops exponentially: by a factor of e (2.71828) for every 50,000 kilometres farther out from the surface you go.
The outer layer of the sun is the Corona. No reactions really take place there, and it is vastly less dense than the rest of the sun. It can reach extremely high temperatures despite that because there is a lot of room for the particles to move around, however the temperatures do not reach that of the inner layers.