Cardio Mix. The best way to lose weight is cardio and a balanced diet. However, do not always do the same exercise or you will increase your risks for stress injuries. Mix things up with running, biking, swimming, or circuit training. Ultimately you need to burn more than you consume. Exercises such as crunches, planks, sit-ups, etc are good for toning the abdominal muscles, but cardio is best for weight loss.
Cardio. Any exercise routine that gets your heartrate elevated above the 130-140's will be the most effective at burning fat and toning muscle. The more you change-up your workout routine, the better you will be at slimming down--your body is smart and will get used to repetitive exercise. Hence, P90X is popular. General rule: consume less calories than you are burning throughout your day and workout.