What is the world's most powerful light machine gun?

Answer 1

Whichever light machine gun fires the most powerful (in terms of force created when the propellant burns and the gas in the bore expands, pushing the bullet downrange) intermediate rifle cartridge.

As far as most effective, this would be a combination of the bullet design, external ballistics (the bullet's ballistic coefficient; how well it slices through air (effect of drag), how much the bullet is affected by wind, etc.), terminal ballistics (what it does to targets on impact), etc. In short, there is no one answer, as every cartridge has its strengths and weaknesses when compared to other cartridges, or even variants of the same cartridge.