To date, Subversion is the single versioning system I have the most experience with. I use it at work, and I was already somewhat familiar with it beforehand. However, with all the buzz over Git over the last few years, it's always been tempting to explore that as an alternative.
I've had a Github account for over a year, but had as yet not added anything to it. However, today that changed. I've had a rather haphazard approach towards my .vimrc and other Vim configuration files for a while, with the result that they tend to be less than consistent across different machines. I've seen that a fair number of people put their Vim configuration files under version control, and that seemed like an effective solution, so I've gotten my .vimrc and .vim into a respectable state and added them to a new repository. Now I should have no excuse for letting them get out of sync.
I have to say, Github is a truly wonderful service. The tutorials for getting started with Git are really good, and make it easy to get started. It's probably one of the main reasons why Git is becoming more and more popular- there isn't really anything comparable for Subversion.