What is the most widely used protocol?

I'm curious to know, what is the most widely used protocol in the world of cryptocurrency and finance? Is it the one that offers the highest level of security, or perhaps the one that facilitates the fastest transactions? Could it be a protocol that combines both these qualities, or is there another factor that determines its popularity? As a professional in this field, I'm eager to understand the reasoning behind the widespread adoption of this particular protocol, and how it has come to dominate the market.