Ubuntu Linux is the most popular distribution but it uses the Gnome desktop by default, which many Windows users may find to be a stumbling block since it looks and acts nothing like the Windows desktop. The standard distribution of Linux Mint uses the Cinnamon desktop, which is much closer in look and feel to Windows, and it is based on Ubuntu so most users can benefit from the technical support of the Ubuntu community.
[Edit: corrected “Linux” to “Ubuntu Linux.” thanks [email protected].]
Unfortunately that uses the Plasma desktop which looks similar but is much more complicated to (re)configure and less like Windows. It might drive a switcher away through frustration.