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.
elementary OS
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.
Debian 8
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
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.
Raspbian
Raspian is based on Debian and optimized for the Raspberry Pi hardware.