What's the best time of the day to workout?

Very simply, the best time to workout is whatever time you can consistently fit into your schedule.

Ideally, you're not exercising in the morning one day, mid-afternoon the next, and maybe squeezing in an evening workout in a couple days.

It's best to get into a consistent pattern.
I generally encourage clients to try to fit exercise into their morning routine. I can tell you that morning exercisers are simply more successful at sticking with their program for the long term. Morning exercise becomes a lifestyle. Exercising before you head off to work allows you to start the day feeling great. You feel productive and ready to take on the challenges of the day.

You may say, "Doug, I'm not a morning person. I have to be at work at eight o'clock and there's no way that I'm getting up at 5:00 or 6:00 AM to exercise." Every once in a while I'll get this response. Fine. Then get your workout in later on. Maybe over the lunch hour or right after work. But I'll tell you that you will generally run into more issues that make it easy to miss workouts if you try to schedule it later in the day. When the day gets busy and push comes to shove, your evening exercise sessions will likely be disrupted.

Here are some facts that will help you pick the right exercise time for you:

Morning exercise allows you to maximize the benefits of something exercise physiologists call EPOC. EPOC stands for "excess post-exercise oxygen consumption". Research shows that exercise increases your resting metabolic rate to varying degrees. People seem to maximize this effect when exercising in the morning.

Lunchtime or afternoon exercise, if it can be squeezed in, will generally help you avoid the mid-afternoon energy dip which naturally plagues humans due to our circadian rhythms.

The only thing that I can think of that's really beneficial about evening exercise is the social implications. Generally evening exercisers are a more social crowd. Want to meet someone special in the gym, rather than a bar? Evening exercise will maximize your chances.

I should also mention that you probably do not want to workout too late at night. Late-evening exercise may interfere with your ability to sleep for several hours after the workout. Of course, if you feel that you need more energy in the evenings, exercising right after work may be a good option for you. Test it and see what works for you. Everyone is a little different in regards to how exercise affects their ability to sleep.