I think Windows XP is the best OS for beginners. My desired OS is Microsoft Windows XP. It is great for the industry and simple to use and easy to learn also. I don't have any complain about Windows OS.
One that fulfills your needs while being cost effective and preferably simple.
So for home/ office I'm using Win7 as it's more compatible with current desktop applications than WinXP or Ubuntu. Last 2 would be my second choice.
In production environment CentOS as currently it's only free os compatible with 3 party applications in my business.
As for security an reliability - any os is as good as you configure and maintain it.