I'd say Git but as Nic suggests - it depends what you're after. We use a mixture of Git and Mercurial in our company.
I find that SourceTree works well with Git repositories held in Bitbucket (if you don't want to use a command line). It's completely free and has built-in GitFlow support.
Whichever way you go in terms of tools I suggest you start by learning the basic functionality of Git using the command line interface.
Atlassian have some good tutorials on their site - so I'd start there: https://www.atlassian.com/git/
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