Depends what I want to get out of my exercise session.
Evenings - I feel too tired and sluggish to do proper training.
Afternoon - only ever exercise in the afternoon to clear away the cobwebs after a long day. Does wonders for the mood, but isn't a proper training session.
Lunchtime - good for a gentle cycle away from the screen if it's nice weather.
Mornings are great for exercise....once I've convinced myself that getting up is a better idea than staying snugly wrapped up in my duvet. I love the fresh air, the sound of birds, the sunrise and the morning sky, the lack of traffic, general quiet and feeling energised and good for the rest of the day about having done exercise already. Oh, and second breakfast after training is the best!