SVN

SVN was designed as a direct CVS replacement, and by now it's done a good 
job of supplanting CVS in most places.  The distributed tools are 
certainly growing fast, but the growth of each is hindered by the fact 
that there are so many of them, so there isn't an obvious one to choose.  
At this point it seems like git, Mercurial, and Bazaar are the top 
contenders, and as a group they might be growing faster than Subversion, 
but not individually.

Also, by now Subversion is mature and well-supported, while the 
distributed tools are just starting to mature and get support from other 
tools.