What is the best time to test your blood sugar?

Depends. If you are on pills and are well controlled, you can check at different times each day - fasting, before or 2 hours after meals. Just keep a record if what time you are checking. If you are on insulin, preferably before each meal and bedtime.
AM and PM. If you doctor wants you to check your sugars daily, you should at least check first thing in the morning (fasting) and 2 hours after your evening meal. Theoretically this will give the highest and lowest sugars of the day. If you take meds or shots with meals, checking sugars before each meal is also important.