Amongst the distributed family currently Git and Mercurial are in the lead.
Amongst the centralized family, SVN is still holding strong (very^2 strong). It is also a default choice in many corporate enviroments. Each of them has their own strengths and weaknesses, both technical and "office political".
Also, on StackOverflow there is a plethora of questions that deal with this subject (choosing a version control system). It really has been dealt to death, shot, resurrected, hanged, then resurrected again (it is currently on intensive care, and every once in a while a new queston pops up ;-), so it might be an interesting read to check out some of the more popular ones. Search under [git], [mercurial], [svn], [cvs] tags.
Here are some of the more popular items, dealing with choosing a version control system in general, as well as some of the mentioned one in particular.
What is the difference between all the different types of version control?
DVCS Choices - What's good for Windows?
Git and Mercurial - Compare and Contrast
Popularity of Git/Mercurial/Bazaar vs. which to recommend
Just these four should bore you to death :-)