I'll recommend Gentoo as well. It gives you exposure to a lot of the system, and there is a lot of documentation. Now, you will blindly follow a lot of guides

I'll recommend Gentoo as well. It gives you exposure to a lot of the system, and there is a lot of documentation. Now, you will blindly follow a lot of guides and not understand some of the things your are changing. But over time you will start to recognize exactly what these tweaks were. Compiling a kernel is a great example. Over time you will start to dive in and tweak the kernel. You will try a non preemptive kernel. You will compile modules in instead of loading them dynamically. (you probably won't notice much of a difference, but you'll do it anyway). Gentoo is a great tinkering environment.