Simplicity, everything is under your control, one of the fewer systemd free distributions, real school to master Linux. Stability, rock solid, had almost no downtime. You can choose what to install, from a very basic (but complete) server with only ssh access, to a full fledged system with all the bells and whistles. It's a 100% hands on distribution. Learned a ton just being using it, and still using it, there's something new to learn every day. Slackware motto is "Keep it Simple", and everything goes accordingly to that motto. Performance wise, it runs in a variety of hardware, from a humble ARM based Raspberry Pi, to a top of line desktop or laptop. What I like the most, is the fact that all my data belongs to me, there's no cloud connectivity to anyone, unless you add it.

It was most stable OS for my services - mysql, apache + php, mIRC servers, perl scripts. I started learning linux with this distro (2005 - 10/10.1 version). Currently I still have 4 servers which are running wonderful - there are no downs since they were installed. I have servers with these versions: - 12.1 - 14.2 - 13.37

Good for learning and for the community.