Morning, lunchtime or evening.
The practical answer is any time is the best time! As long as people exercise around three times a week then they should be quite healthy.
However, the most efficient time is the morning. The reason for this is that your metabolic rate is highest in the morning. For a few hours after you have had a workout, your body continues to burn calories at an increased rate. Thus, if your metabolic rate is highest in the morning (which it is for everyone!) then your body will burn more calories after a workout in the morning than at any other time of the day.
Having said that, I go to the gym in the evening as that's the only time I can make it. It's probably the worst time in terms of efficiency so I just a bit more than I would normally.