At the point of the kenobi series, Vader and Obi-Wans amounts of midichlorians could be pretty similar but Vader has a far better control of all of his midichlorians. The amount isn't what decides how strong in the force someone are, but the control over them is. Still a high midichlorian count can still help determine how strong the potential connection to the force can become, but what 

Plagueis:

This is why I would say Plagueis is one of the most powerful force users, because he spent half his life studying and manipulating midichlorians in himself and in others. He devoted all his time to understanding them to the point where he could command these living organisms to do as he wished. In the end he could even choose to "deactivate" them in others in order to kill them without physically harm them. 

THEORY:

This is also the foundation for my theory that he could still be alive in the galaxy somewhere. When Darth Sidious betrayed him and killed him, I think he could be able to manipulaet his midichlorians to "feign his death". Deactivate them to the point they were all almost completely shut down, giving anyone studying him the impression of him being dead. Then later when left alone he could activate them again and escape, leaving the galaxy to believe he was dead. 

Let me know your thoughts on this!