What is the best exercise workout to lose weight fast?

It's not that simple. It is possible to lose weight by exercise only, but the high calorie foodstuffs that we have available today makes that approach really hard. Something like a single cupcake will take about 45 minutes of dedicated workout to use up - so in reality, you really have to start by controlling your intake.

The amounts of calories that you use up is closely related to your heart rate, so the best exercise to lose weight is the one that strikes the best balance between getting your heart rate up and letting you keep it up for, say 45-60 minutes or so. Exactly how you're exercising is far less important.

Suppose exercise A boosts your heart rate with 100 beats/minute, but you're only able to keep it up for 5 minutes. That's 500 extra beats, not much to brag about.

Then there's exercise B that boosts your heart rate with 80 beats, but you're able to keep it up for 30 minutes. That's 2400 extra beats, a far better deal.

Which exercise that's the "best" is impossible to say, as it's dependent on your fitness level/swimming skills. Any exercise that you can do hard enough and long enough to get that sustained elevated heart rate required for a good calorie burn will be the "best" exercise.