Visual Studio 2015 file comparison tool can be considered good enough for most of the use cases. Before learning how to compare files with Visual Studio, I used to use WinMerge, or another tool to compare. Not anymore of that!

What is still missing in the IDE is a small menu that allows us to select 2 files and compare them. From command line, you can do it with devenv.exe /diff which compares two files. It takes four parameters:
SourceFile, TargetFile, SourceDisplayName(optional), TargetDisplayName(optional)

You can launch it with below command:

devenv.exe /diff “e:\source.txt” “e:\target.txt” “SOURCE” “TARGET”

But within the IDE, things change a little. Instead of launching the command from the Command Prompt, we can do it from the command window with a syntax of style

Tools.DiffFiles SourceFile, TargetFile

which makes it more easy to use.