I prefer to use CentOS, mostly because I use RHEL at work, so at home I write scripts to test out on my CentOS VM before I migrate them over to my machines at work.
If you want to get some decent experience start with no GUI at all (Ubuntu Server is a good starting point for this), then attempt to download your GUI of choice. Try to compile it from source, install it, and configure it. This will give you decent insight as to how the X-Window system works, what can go wrong (alot can go wrong) and how to fix it. Most of the problems I see at work with Linux involve X and problems with the GPU.
As others have said there really is no best distro, just find what suits your needs and tailor it even further until you find a good fit. Ubuntu is a great choice because of the large community out there, just experiment and have fun.