What is the best way that steak should be eaten?

The customer is always right in matters of taste.

I prefer medium rare. I’ll eat medium rare.

If someone wants theirs blue rare or well done, it doesn’t harm me at all. So more power to them.

This is a simple example of John Stuart Mill’s “Harm Principle” ethics. As long as it doesn’t harm someone else unreasonably, it’s probably a morally permissible choice and our collective systems shouldn’t restrict it.