I think geofft's points go a long way. I think having this conversation would require a lot though - what is "Secure" what is "general purpose". Is OpenBSD even general purpose? Is the base install general purpose? What makes an OS secure?

I think maybe as a "with 0 configuration to the OS/ services" OpenBSD could be a top contender. With "I have practical security challenges to solve and I'm willing to change things in this system" I might choose Linux. If I'm handing a laptop out to an employee or friend, maybe a Chromebook, or even Windows! It's a pretty nuanced discussion that I couldn't do justice to without really caring and feeling that everyone involved in the conversation is equally invested/ coming in with the right attitude, which is impossible on an internet forum.

Well, there is only one real "general purpose" OS that runs on supercomputers, mainframes, servers, laptops, tablets, mobile and embedded, and that's Linux. So it's also the most secure.