All depends on what you want to learn and how much breakage you're willing to deal with.
Everyone on here (myself included) is likely to have a certain bias for their own preferred flavor. Check out distrowatch if you haven't already done so for reviews and information on any of the suggestions offered.
Linux has come a long way from when I started using Slackware back around 2002. From there I went to Kubuntu, Sabayon, Mint and now Manjaro (Cinnamon) on desktop / laptops. Work has been mostly Ubuntu and Debian with some centos and centos-based products (xenserver) in the mix.
To come back to your question, it depends on whether you're looking to learn from a work-skills perspective or personal curiosity. If you want to do a real deep-dive you could do Linux from Scratch, Gentoo or Arch....
My personal recommendation (bias included) is to run Manjaro Cinnamon and use KVM or VirtualBox to experiment with other distros.
Reasons:
- Rolling release is awesome
- It's close to bleeding edge, but still very stable
- AUR is like PPAs but so much better.