The best time for a coffee is before a work out, with the preference being black coffee or an espresso.
There is an abundance of evidence showing that coffee can increase fat burning during exercise, make you more motivated to work harder and improve your work out capacity.
Drinking coffee post work out will delay your recovery and elevate your stress hormone (cortisol) at a time when your body needs to clear it. Exercise naturally raises cortisol by overloading the muscles so the body has to adapt by building more muscle tissue.
After training it is in your best interest to lower your cortisol levels as quickly as possible.
A much better alternative to coffee post work out is green tea.
Green tea has numerous health benefits, one being that it is full of antioxidants that can accelerate recovery, and it has been found to have no effect on cortisol levels.
If you are a big coffee drinker and can't go without it post-work out, try decaffeinated organic coffee as an alternative.