The best time of the day to practice usually depends on your personal clock. In order to get the most out of your practicing you need to be relaxed and focused.
So in regards to what time of day is best for practicing, that needs to be at a time of the day when you are not absorbed into your own thoughts, this keeps you relaxed, and it needs to be at a time of day when you have the energy in order to remain focused.
For me personally I find the morning and the middle of the evening to be the two best practice times because that's when I'm relaxed and not too tired. However, there are times where I've found myself practicing at different times then those, too. It all comes back to your personal clock.