Over the years, I have tested several Linux distributions either on the desktop or as servers (in the meantime, I have stopped distro-jumping and have settled exclusively on Ubuntu-derived distributions for the desktop). Here is a small recap of that experience.
The venerable grandfather of many Linux distributions (although Slackware is slightly older). It remains my preferred distribution for any server installation (even on the oldest devices, in my case a Pentium with 64Mb RAM - without GUI), but I have preferred its Ubuntu derivatives for desktop use.
Ubuntu was historically my distribution of choice until the whole desktop environment debacle brought us Gnome 3 and Unity. While Gnome is more mature now and Unity was discontinued, at the time this led me to XFCE, Mate and later Cinnamon. KDE clearly has its strengths but never truly seduced me.
A relative newcomer (2011) in this list, Elementary OS is based on Ubuntu but has evolved in a very sleek, if somewhat Mac OS X inspired, alternative. I used it for a while in the Luna/Freya early days but dropped it due to various stability issues. Besides, while built-in applications are indeed very well integrated with the OS, this is not at all the case for other applications, which defeats the purpose.
A even younger distribution (2015), Solus OS stands out for 1) not being based on another distribution, 2) having adopted a rolling model and 3) showcasing the praised Budgie desktop. Still to be tested...
Crunchbang was a very lightweight Debian-based ditribution using the Openbox window manager. It is discontinued as such but still exists as a community continuation under the name BunsenLabs. An Arch-based variant also exists (Archbang) but like all Arch distro it never inspired me much.
Slitaz is a very very lightweight distribution which would work on all but the oldest PC. Very much recommended also as a maintenance or rescue tool, on a bootable CD or USB stick.