What is the most common subnet mask?

In the realm of networking and specifically IP addressing, could you elaborate on the significance of subnet masks and why they are crucial? Furthermore, as a technology enthusiast and frequent user of computer networks, I'm curious to know - what is the most commonly used subnet mask in today's networks? Is it the Class A, B, or C default subnet mask? Or has there been a shift towards more specific, non-standard subnet masks to better accommodate modern network architectures? I'd appreciate a concise yet informative answer that sheds light on this topic.