There are several, depending from what one wants. TrueOS, formerly known as PC-BSD, is a good entry point if one wants a desktop-style system. Or one can try vanilla FreeBSD. Or OpenBSD. Have some user guides and Handbooks to read:

* NetBSD Guide: https://netbsd.org/docs/guide/en/

* FreeBSD Handbook: https://freebsd.org/doc/handbook/book.html

* DragonFlyBSD Handbook: https://www.dragonflybsd.org/docs/handbook/

* TrueOS User Guide: https://www.trueos.org/handbook/trueos.html

* PC-BSD User Guide: http://web.pcbsd.org/doc-archive/10.1.2/html/pcbsd.html (viewable off-line directly in both PDF and HTML forms in /usr/local/share/pcbsd/doc/)