My favourite time of year is an elusive period in November; it occurs after all the leaves have fallen and the grass has yellowed, but before the snow falls. On a mild, cloudy day, it creates such a calm, quiet, peaceful scene. We don't always get this; winter sometimes comes early and it snows before the right conditions for this are met. I suppose the fact that this time of year isn't even guaranteed helps make it that much better when it does manifest.
More generally, I have to say mid-fall and early winter are my favourite times of the year. Saying October to December would be a safe call, assuming we don't get a frigid winter kick-off (which does happen, sadly).