The best time of day to workout is the time of day that fits into your schedule best. Seriously, the most important thing about any exercise program is that you do it and you stick with it. If you do not make it easy to fit into your day, you will eventually start skipping it because you day is just too hectic and something has to go. It really is a personal choice.
Scientifically, there have been studies that prove that your body responds the same to exercise no matter what time if day you do it. The important thing - to quote an old Nike ad campaign - is to "Just Do It". There have also been studies that prove thay the temporary metabolism boost you get from a workout happens no matter what time of day you exercise.
Some people find that working out in the morning is easier because if you wait until late in the day, more things come up that tempt you to skip the workout. Very rarely do you have a surprise schedule change that occurs at 7am, but lots of schedule changes happen later in the day. And for some people the idea of getting up earlier to exercise is horrible and they could never do it.
Some people find that exercising late in the evening "pumps them up" for a few hours after and they have trouble sleeping. And some people have the opposite effect - exercise relaxes them and makes them a little tired so they sleep better.
I prefer to workout late afternoon during the week, 4-7pm ish, but I am flexible. If I have something I have to do during my normal workout time, I rearrange and squeeze a shorter workout in some other time during the day. And on weekends I prefer to get my workout in first thing in the morning. During the summer when I workout outside, my weekend morning workouts have an added bonus - enjoying that peaceful time early in the morning after the sun comes up but before the rest of the city wakes up.
Just exercise whenever you can fit it into you schedule best, and do not be afraid of experimenting to find what is best for you.