This post isn't to advocate DiffMerge over another file comparer, but an explanation on how to set up DiffMerge with either TFS in Visual Studio or Git.
I've been using DiffMerge since VS2010, so pretty much forever as far as I am concerned. Back then there was no built-in diff viewer in Visual Studio so you had to use a third-party tool. Because I've used it for so long I'm quite reluctant to change. Looking at differences in files is something every developer does often, whether it is local code changes or a code review for another member of the team. It has to be a quick and efficient process. That's why I stick with DiffMerge. I know it works, I know how to use it, I'm familiar with the layout, and I can quickly navigate and scan the document.