The best Linux distro for a sysadmin is the one that is needed to get some sort of "real work" done in a production environment. The sysadmin isn't the important part, but rather the servers she takes care of and the work they do for the organization. Whatever distro best accomplishes that work is the "best", which varies substantially from situation to situation.
That being said, if you want to learn to be a sysadmin, I'd suggest (in no particular order):
- CentOS (free rebuild of RHEL, which is widely used in industry)
- SuSE (SLES and SLED are also popular in the corporate world, especially in Europe)
- Debian (believe it or not, I know several shops that have a most or all Debian infrastructure, and most of the skills transmit well to Ubuntu server too)
- FreeBSD (not Linux, but popular on Web server farms)