I've tried a lot of Linux distros over the last 13 years. I even did a stage 1 Gentoo installation, which I don't think they support anymore.

Instead of reeling off a list, I would rather ask you what you hope to learn from a different distro. We all have enough spare computers and spare hard drives to install a bunch of stuff and never touch it again. In contrast, what would you want to keep at the end of a couple months?

For example, lots of folks have mentioned Gentoo. It's easier than ever to set up a fresh Gentoo box, because multi-core machines are faster than ever at compiling. However it's just as much a pain to update when you have to compile X repeatedly or leave it behind.

Nevertheless, Gentoo has one thing most Linux distros cannot offer as readily: out-of-the-box PXE and TFTP server setup. If you've been wanting to learn more about bare metal provisioning (a useful skill), Gentoo is the place to start. There is nothing quite as rewarding as the first time you trick one computer into installing a setup on another computer.

If instead you want to learn more about keeping a server running while meeting business compliance, then I recommend CentOS. It's the open-source version of Red Hat Enterprise, which has a completely different daemon configuration system (one that I find a lot easier to use). Anyone can say "eww, it's not Debian-based so it must be evil". It takes only a few hours of exposure to chkconfig and some of their other command-line tools to realize, "wait, it's working and it's tight. I might accidentally get something done this afternoon."

Crunchbang has something esthetically pleasing: cleanliness with a desktop. I recently installed it on a Pentium III 866 MHz laptop from 2000 or so and loved that it worked without any special effort and I could customize the desktop using only vim. While I had to download a 32-bit non-PAE version for a first-gen Centrino laptop to run it, I got that ISO directly from two clicks off their front page.

Building, running services once you've built them, tweaking your interface to these events... they're all different goals. Heck, you may be more interested in trying Linux on different CPU architectures to learn how they differ and what that means for maintenance.