Linux. The Mint Cinnamon is the variety closest to Windows and is almost seamless in the learning curve if you already know how to use Windows.
It's free, you can install alongside Windows and boot either OS at startup if you choose, the vast majority of software is free including an entire office suite that is compatible with Microsuck formats, choice of browsers, very fast, very secure, doesn't try to take over your life/identity and monopolize your finances and sell personal information to the highest bidder like Microsuck and Crapple, doesn't use your computer's resources and processing power as a bot to process information for outside computers like Microsuck, etc. etc.
If you are needing a computer for Internet browsing, office applications, paying bills, etc., Linux Mint Cinnamon is the way to go. If you are into high end CAD programs or video games designed for Windows, then there's the dual boot option for starting Windows when you need it.