Sourcetree has been my favorite, followed by the default GUI installed from the git-scm.com. I've not been a fan of the VS extension because of performance (a want NCrunch and Resharper to have all my free cycles).

Visual Studio Git Tools in 2012 and 2013, and then Atlassian SourceTree for the stuff VS can't do (VS's integration is based on libgit2 and has limitations)

A lot of our developers use GitExtensions and like it. I don't personally like or use it, so I am not sure why it is preferred.

TortoiseGit is pretty decent, though it doesn't have VS integration.