The answer is "None".
It is not possible to create a projection of the world which preserve the scale in all direction from any point of the world.
The cylindrical equidistant projection, you only have true distance along the meridians and along the equator. So if your points don't have the same longitude, you get it wrong.
With an azimuthal equidistant projection, you would need to center it on every point from layer in order to have true distance.
So, my recommendation is to use Python in order to get your "great circle" distance. You can see this post based on the geopy package. First you compute the geographic coordinates of your points (calculate geometry), then you measure your distances in a loop.