I think the CRUX is LFS with stage 4. The system is simple, there are no superfluous entities (It may seem Spartan at first glance, but this is compensated by complete control over the system from the first steps)
If you want to understand how Linux works, this distribution for you.
Version: 3.2 Rating: 10 Date: 2017-02-16 Votes: 5
This is a great distribution for advanced users, allowing to create a system dedicated exactly for a server you use. It compiles all the packages and creates packages for the further uses and does it in pretty intuitive way. One can also learn a lot by using it. Recommended for advanced (or advanced to be) users.