Regardless if your goal is to lose fat or build muscle, all workouts should start with legs/hips/back work. It's interesting because these are the areas that most gym goers avoid like the plague! If your effort is put into these areas first when you're fresh, it not only allows you to lift heavier but it serves as a complete warm up for the rest of your workout. Hard leg/hips/back work gets your heart rate through the roof and just flat out stinks! All the more reason to do them first and get them out of the way.
Here's an ideal way to organize your workouts: