The problem lies in not what you want to learn, but what you want to get out of it and your ulimate goal for learning it. For linux distributions there are the three main (well only) pure distributions:
Slackware: Arguably the most Unix like of the three. Installs are surprisingly easy if you do the full-install of DVD 1, which allows most anything you would need to work out of the box and gives you a solid desktop or server depending on your choice. Custom installs (those wanting to purpose build) can range from slightly challenging for the uninitiated to an outright confusing pain in the ass if you have never done it before- however, here lies the path to learning. It is suggested that you do at least one full install and get to know slackware under the hood before attempting custom jobs. The documentation is excellent, probably the best of any of the systems (free too), the Slack Book will be your best friend here. It has been said before that if
you can learn slackware, then you know linux. My experience thinks that this is true.
Redhat: As far as I have read, generally used for server environments or anything needing "hardcore computing" (slackware is just as good as this but not as common due to commercial interests unfortunately). This is another base level system and is not generally recommended if you want a good desktop environment, although it can make an excellent one if you put the work into it (i.e. Fedora) Documentation is lacking as this is more for commercial interests and support here costs money. The Redhat books if purchased are massively in depth and not for the faint of heart but are another excellent way of learning linux. Redhat is generally recommed to learn because there are many companies that use this system, so employability is a little easier though others have different experiences.
Debian: Arguably the easiest of the three for beginners. Package management is excellent here as far as ease of use and has an excellent support community. Without bias, this is generally my distribution of choice for most tasks. Installs are simple and are capable of producing a very streamlined desktop environment out of the box. However, the cost of this ease of use is some amount of automation that makes it difficult to actually learn what makes linux tick. If you are looking to simply use linux this is an excellent choice but if you want to learn, you will need to have some motivation to dive in to the system.
Each of these main distributions also has an assortment of desktop spinoffs. Some examples are: Salix and Porteus (Slackware), CentOS and Fedora (Redhat), Ubuntu, Linux Mint, and AntiX (Debian)
Two other Linuxes to try are Gentoo which is a source based distribution which for you would allow excellent learning potential on the software level as you compile, make and install programs from source code (or with their package manager, pacman or portage I can't remember). Another good learning experience is Linux From Scratch/Beyond Linux From Scratch which is an operating system which you build from scratch ( www.linuxfromscratch.org
). The nice thing about LFS is that you put the OS together piece by piece from toolchain to libs then the kernel and bootloader then onward to usable programs and X windows if you want. Also head over to kernel.org and read some docs as well seeing as the Linux kernel IS Linux, I wouldn't get too in depth at first but it is essential that you understand what the kernel does and how it is used.
All things being equal I would suggest the following, but make sure that you know what you want to get out of this experience and set some goals: If you are looking for employability/marketability the majority of companies like RHEL, Slackware is excellent if you want to learn how to be a sysadmin or are looking to build servers due to the well rounded learning curve, I would avoid Debian if you are looking to learn simply because many things can be taken for granted or overlooked because you don't realize that they are happening, Gentoo and LFS are excellent choices for a more academic learning approach and are also great segways into Slackware because you will be familiar with filesystem hierarchies and command line syntax and commands.
Also see below for the LDP