Each season has its own advantages and disadvantages. In my opinion, they are all equal.
Of course, the weather is warmer in the spring, summer and fall. But, depending on your location, spring and fall could be very wet. In summer, on some days, it can get too hot to work outside for long periods of time.
In some cases, winter may be ideal. Heat isnt a factor and usually the cold isnt much of an issue. when we do second story additions, its easier for us to deal with snow than rain.
Again, I think it depends on where the contractor is located.