Who Has the Final Word in Halachah?

I was once teaching a Mishnah B'rurah class to some beginners. I was asked:

"Isn't it true that S'phardim Always follow the m'chabbeir and that Ashk'nazim Always follow the Rema?"

I said, "While that is generally so, it is not absolutely so."

The same for prefering Bavli over Y'rushalmi, which is the default, but not absolute.

And so is often the case, rules have exceptions. I call them "defauts"

The Bet Yosef set up a Bet Din, to determine Halachah which he himself ignores in numerous cases.

And so does any one here know of anyone who follows a given "Poseik X" 100% of the time?

Best Regards,
RRW