I'd recommend Subversion, simply because of its pervasiveness in open source projects. There's definitely a lot of opportunity to put your Subversion knowledge to use in open source projects, and if you do, you'll learn a lot about coding, working as a team, and about using a VCS properly in a team (which is half of learning to use a VCS, and is the hard part because you can't learn it very well on your own). Fewer open source projects use proprietary VCSs, which means you'll get less of an opportunity to use them in the real world unless you're working specifically with friends or as a job. I haven't used the other two you mentioned, but I have no complaints about Subversion, it's quite straight-forward to use.
I'd also like to recommend a distributed VCS, such as git. With a distributed VCS, since every participant in a project has a complete copy of the source repository, and has complete control over their own copy, it means that you're given greater flexibility to use the VCS capabilities. You'll find that you have the freedom to commit as often as you want, and create new branches whenever you want, which is great when you're making changes to code and you want the ability to go back to an old version if necessary. A distributed VCS lets you use the VCS as a tool to help your coding, rather than simply as a way of storing and sharing your code with others. It's also very easy to set up a new repository in a distributed VCS. With git you just run git init and it'll set up version control in the current directory. How easy is that?