Some say that working out in the morning is a good way to get your blood flowing in the morning and that it will give you more energy for the rest of your day. Some people say it runs them down. I think it depends at least partially on your own daily rhythms.
I am not a morning person, so I tend to miss too many workouts if I aim to do them before work. I just can’t get motivated in the morning, especially when my bed is soooo comfy. Instead, I go every day after work while I wait for the gal who carpools with me to finish work. I just started going regularly again at the beginning of this year, and I’m already at the point where I miss it on days I don’t go. (Yay! One battle won!)
Personally, I’d say that the best time is a time that you can regularly commit to. Consistency is more important than time of day.