Most effective boxing style

You must adapt your style depending on your opponent. This will be Mayweathers downfall.

Well defense is almost always applicable, while offense can come and go. Guys who are heavily geared toward offense are up a creek without a paddle if their offense fails, a more defensive minded guy even if their offense isn't working they have their defense to try and create some offense or possibly use it to frustrate a guy into mistakes. It is just a matter of options really, having more two ways to generate offense is better than only having one. That said having one exceptional option is better than having two of three mediocre ones (at least most of the time).

There is no one style that's most effective. It all depends on how good a fighter is at the style they fight.