Control version systems

CVS, Subversion, GIT, Mercurial... I need to see and choose from them.

We are a small business creating websites and we have to implement some control version system.

Which could be better for this situation?

Also, which are easier to install and learn?

Google Docs. You know what they say, "given enough eyeballs, all bugs are shallow". Well, with Google Docs, you have the eyes of Google employees, the NSA, marketers and advertisers all looking at your code.