If you are going to do low intensity stead state (LISS) like Brick mentioned, then AM is your best bet. This way you glycogen stores should be low at that time and once they are used up, you use stored fat.
HIIT, although personally I have found it best in the AM just for effect it has on elevating metabolism throughout the day, can be used at any time because the fat burning from HIIT mainly comes from the EPOC effect. Your metabolism is elevated for a prolonged period based on the intensity of the cardio and you'll burn fat throughout the day regardless of whether it is done on an empty stomach or not.
In addition to caffeine in the AM, adding in yohimbine HCl before AM cardio is also a great way to burn fat from stubborn areas. For those unfamiliar with yohimbine: Yohimbine is an alpha 2 receptor antagonist, which means it inhibits the action of the alpha2 receptor. Alpha2 receptors store bodyfat. Therefore, in order to burn stubborn fat we must override the inhibitory effect of the alpha2 receptors. By blocking the alpha2 receptor with yohimbine, the negative feedback caused by norepinepherine binding to the alpha receptors is reduced and fatty acid liberation is increased. So basically, it makes it easier to burn fat from the areas that you tend to be trouble spots for fat loss.