You'll hear different things from friends--and even trainers--but the correct answer really depends on what your goal is.
If it's weight loss you're aiming for, a morning workout may work best, since a.m. exercise revs the metabolism and helps you burn calories all day. Plus a workout sesh first thing also reduces blood pressure and helps you sleep better at night.
Additionally, you're more likely to stick with a morning regime because there are fewer distractions first thing in the day. But if you want to boost muscle strength, you may want you hit the gym between 4 and 6 pm. According to research, our muscle strength tends to be lowest in the mornings, and becomes better as we go along our day.
Researchers also found that evening-hour workouts led to greater gains in muscle size. Plus, working out after work is a great way to tackle the day's stress (and, if you don't already know, increased levels of stress can lead to weight gain).