What's the best exercise to get a flat stomach?

Ah, the age old question that every personal trainer is asked time and time again. You could do crunches until the cows come home, but the truth is that if your diet and lifestyle aren't doing you any favours, your efforts will all be in vain. Depending on the position of your hips, there may be some abdominal exercises which are unsuitable for you. Leg raises can be very tricky for any women with an anterior pelvic tilt. The spine needs to be in complete contact with the floor to execute this exercise correctly, however, if you have an anterior tilt you will naturally have a gap between the floor and lower back.
Tips for a flat tummy:
Choose fresh ingredients and look for foods that have proven benefits for reducing abdominal fat. Dark green vegetables such as broccoli, kale, spinach and brussel sprouts have been and focus on lean proteins and complex carbohydrates, with the majority of your plate filled with the veg.
Ensure you are getting enough sleep as sleep is vital for the body to repair. Lack of sleep can cause poor concentration, increased risk of obesity, diabetes and heart disease. It can also prevent the body from expelling toxins and waste.
Focus on free weight exercises that will force you to use your core muscles to balance and use the whole body while you train. Fixed path machines do not allow you to use your whole body and can actually make muscular imbalances and tightness worse. Kettlebell training is fantastic for toning the mid-section without the need for hundreds of crunches.
Consider a pilates class. Pilates doesn't just tone the external abdominal muscles, but focuses on the core in its entirety. The benefits of pilates make it hard to beat. It encourages us to have a strong trunk to strengthen the body as whole. And, provided that we work hard when we train and focus on a clean diet, a flat stomach will surely follow!

One thing I was told when I was learning was that '6packs are made in the kitchen not the gym'
Basically all the crunches in the world and you wont see your abs unless you can lower your body fat %
So do your crunches, leg raises etc, but dont neglect your cardio routine and look to keep your body guessing by making changes every few weeks. Also keep an eye on your diet for those hidden fats!

This is simple and very easy to obtain provided you are willing to put some effort into achieving your goal -- Flat Tummy.
Hard Work - Eating fresh food (cut all processed foods, sugars or anything with sugars, saturated fats) - and then just work towards it and enjoy the journey.
Fitness tip - Work the whole body - keep your workouts short and intense.
Sample session: Tabata Protocol 8 sets 20 seconds work 10 seconds rest Body weight only
5 exercises
Lunges
Press-ups/Push-ups
Hip bridges
Crunches (with heels close to your butt)
Plank
I hope this helps. All the best with your goals.

The best way is diet and exercise, the hanging knee raise is a good exercise for the lower stomach. High activity training with minimum rest periods will cause weight loss. Heart and lung action are very important to emulsify fat. The stomach has a good blood supply so the abs do not have to be worked with loads of sets of high reps, simply work the abs 8-12 reps with 4-5 sets per exercise ,good luck its worth it in the end.

It's best to use a mix of exercises as most exercises only target one part of the abdominal area. Assuming that you have no equipment the following exercises will get you toned up in no time.
Crunchies for the upper abs, leg raises for the lower abs, spinal or russian twist targets the obliques and not forgetting the good ole cycle crunch. I would do 20 of each exercise and then repeat, that will give you a great abs workout.
Crunch: feet on the floor, heels off the ground, fingers touching your ears and elbows out. Slowly crunch up and then down, make sure it's nice and slow, you can hold for a few seconds at the top to make it a little harder.
Leg Raise: On your back, legs start straight up in the air, hands under you bum, slowly lower your legs DO NOT touch the ground, if you feel your back start to arch, go no lower! slowly raise the legs again
Spinal Twist: On your back, legs at 90 degree angle (think seated position but on your back) put your elbows out to the side to give yourself more stability, slowly lower your legs to one side, slowly raise and then lower to the other side.
Cycle crunch: Lie flat on the floor, hands beside your head, bring knees up to 90 degrees (think seated position, but on your back) This will be your starting position. Simultaneously, Bring your right knee closer the left knee further away, while doing so crunch up and bring your left elbow to your right knee. Go back the original position and repeat on the other side, bringing your right elbow to your left knee
Happy crunchies!

Many people think that they can have flat stomach just by doing same abs exercises or wearing some fake "zones" which make them swet at the particular muscle.In my opininon
if you want your belly to look nice start eating healthier .This is the most important.After you get rip that extra fat you already have,your muscles (not only over the stomach) are going to get on surface.Thats where you need to start exercising on the particular muscle depending how you want them to look.Cardio ,about 35-30 minutes every day including some abs exercises 3sets 25-30 reps with 20-30 secs rest bettween sets (in my opininon super-sets and circlular training are best).Avoid eating to much Carbs ,and forget eating before you sleep ,except maybe a glass of milk.In a few words thats what i have to suggest..