Using mercurial -- it and git seem significantly better than the rest, but when I recently test-rode them both, mercurial felt like it worked better cross-platform, and the overall design felt cleaner. Git would be my second choice, and I'm sure github and the network effect will do a lot to popularize it.

(I've read several blog posts in which people try git and are absolutely thrilled, but in almost all cases they were previously using CVS or SVN.)

FWIW, I had been using Monotone for a while, but mercurial seems quite a bit simpler in day to day use.