I think the "most efficient" and effective way to do good is to look around near you, i.e. to see first-hand the issues and the effect of your actions.
I agree with this and similar posts suggesting they find local non-profits. There's probably no perfect answer to what's most "efficient" and I think that's probably the wrong way to look at it. As a single, non-billionaire, there's only so much you can do to help others. But, you will get much more satisfaction from seeing "first-hand" the "effect of your actions". Sending your money to some faceless global charity…
There're [nominally] 8 billion people on the planet
A billion bucks is only twelve cents per person