Git.
Because it's the first and only DVCS I: got interested in thanks to LWN / learned / found
plenty of good documentation for.
As far as I'm concerned, I find it cleverly implemented (totally fits into the "one small tool
for a given-task" Unix mentality), find it's interfaces easy to learn and easy to combine with
other Unix tools, and thus like to use it wherever possible. I have no idea about the other
DVCS', whether they are similar enough, but I also positively love speedy applications, which
makes me stick to Git for the foreseeable future.