Swing Speed is the number one decider when it comes to picking out what Flex works for you. In simple terms, the slower you swing the club, the softer the shaft you need. This is because the softer shaft will "whip" the head quicker and allow the club head to do more of the work.
When you have a high swing speed, a stiffer flex shaft will allow you more control, as the burden of distance is being transferred more to your body. But that isn't to say stronger players need to play stiffer shafts. There are so many myths around shaft flex, and the only thing that is for certain is that you should get fit by a professional.
In a professional fitting, the club fitter will measure your swing speed and then have you try different shafts. This will allow them to make solid recommendations about what clubs and shafts you should be using. Don't have the ability to get fitted by a pro? Then try to borrow a few different flex shafts next time you hit the range. Golf is a game of touch, and the more you try, the more you will know what you like