The answer is NOT sit ups or crunches. Or actually any kind of specific ab work.
I used to be asked this questions CONSTANTLY when I worked at a commercial gym
And every time I loooooooooved telling chicks the truth
I mean I looooooooooooooved seeing their faces drop
As these tend to be the girls who look after themselves, care about their diet to look good and then come to the gym to do fitness classes, 18 kg leg presses and loads of crunches 'to try and lose that last little bit of belly fat'
They couldn't be doing it any more wrong (well, don't get me wrong, going to the gym to do silly things is better than sitting on their bum watching telly)
Low intensity steady state (LISS) cardio actually encourages your body to put the weight on around your mid section WAAAAAAAAAAAAAAA
So the best exercise for belly fat? You ready for it?
If you're working on machines (safest option for beginners), the answer is leg presses. HEAVY leg presses. YEP!
The reason is that doing them you're using the biggest muscle groups in your body
And using the biggest muscle groups burn the most calories
And when you maximise calorie burning, you're burning more fat.
It's important to note here than you canNOT target fat loss. So there is no point in doing ab work to try to lose belly fat, as it'll only make your abs stronger
AND! Here's another shocker - doing ab work correctly, in the effective way, will create hypertrophy in your abs - i.e. make them GROW, so doing targeted ab work can actually make your waist slightly bigger! Then if you add incorrect breathing in whilst doing ab work, you'll even further grow your waist.
So get on the leg press, do 2-3 HEAVY sets of 8-12 reps to burn the most calories
Because when you've really worked the muscles, your body will keep burning calories a day after your workout. And the more lean muscle you have, the more calories your body will automatically burn in a day.
There you have it.
P.s. If you're using freeweights, deadlifts and squats are your biggest calorie (belly fat) burners as you'll be using pretty much every single muscle in your body doing them.