What is the best time of day to exercise?

Metabolically, there is no difference. It doesn't matter as far as burning calories whether you exercise in the morning, noon, or night.

Physiologically it is better to work out later in the day. Your joints are lubricated and muscles are warmer and contract more efficiently. Don't wait too late though. If you exercise at night, sleep could be interrupted if your session is too hard or too close to bedtime. People differ on this, so you'll have figure out what works for you.

However, morning exercisers have been shown to stick to their programs better than those who put it off. The later in the day you wait, the more time you leave for life to "get in the way."

The easy answer is that the best time of day to workout is the one that you will do!