Which pokemon anime is the least popular (worldwide)?

We know the most popular is the OG(pokemania) followed by kalos, at some point of journey it's reached as peak of a popularity as XY.

But which is the least viewed one, I feel like it's should be black and white, but is it? And what about the current anime, horizons preforming really bad in YouTube, but that's understandable, but is it the least viewed or is it some other one I am missing?