About vanilla systems, I think no one can make a worse job than Microsoft. When I discovered that Windows XP was auto-executing some files in any USB stick you plugged in it, I decided to not take Microsoft seriously EVER about security. Yes, it was after their grand announcement they would focus on security. They probably improved many things since then, but I can't trust a company that did not understand the problem about arbitrary code execution during all these years to do a half-decent job at security.

The world-wide epidemics in computer virus would not exist without Windows. It is not about it being the most prevalent OS : Linux is prevalent amongst webservers, highly valuable targets. iOs is the most prevalent OS on cellphone, always-on targets. Neither see virus spreading. Think about it.