I generally recommend exercising first thing in the morning so that you are able to exercise without having interruptions. When are you going to train most effectively? When are you going to be able to put the most effort in? When are you going to have the most energy? When are you going to be able to train most efficiently and effectively unencumbered by a busy gym or distractions on your phone?
The simple answer is to exercise whenever it best fits your schedule or your energy level.
Some people are early risers and can make time for a workout before they start their day, while some have more energy at the end of the day. For the general public, the best time to exercise is whenever you can do the workout.
Since many companies now have a gym available to their employees, a lunch time workout may be a good option, as long as there is a place to shower before returning to the office.
Exercising before going to sleep may cause one to be energized. If you leave exercise for the end of the day, there is also a chance that something may interrupt or prevent you from working out.