I am not going to say these are the best or “must have” Linux distributions but they are some of my favorites and what I use.
Dare I say the sexiest distro available, based off of Ubuntu it has a very OS X feel with simple clean applications and not a lot of bloat out of the box.
One of the oldest and highly used distributions, Debian is composed entirely of free and open-source software and is the base for many other popular Linux OS’s such as Ubuntu. Supporting multiple desktop environments, my favorite desktop environment (for Debian 8) is Xfce.
CentOS is a free, enterprise-class, community-supported computing platform which aims to be functionally compatible with its upstream source, Red Hat Enterprise Linux (RHEL). Used more as a Server platform than desktop, my personal use is almost completely shell only running server applications.
Raspian is based on Debian and optimized for the Raspberry Pi hardware.