The best part of commuting for me is getting on my bike and going for a bike ride each morning. I love being outside. Riding gives me a mental alertness that I love. Though I have to say this time of year is not my favorite and it has nothing to do with the cold, it has so much more to do with being dark in the morning and not being able to see the scenery. I love all my different routes to work.
My favorite part of my commute is pick a day, I am sure it is something different. How the light hits the colorful leaves this time of year in the evening. The long downhill on a very rural part of the ride. Seeing the lake between the trees.
Though I can tell you the least favorite part of my commute is the first uphill when I leave work in the afternoon.