The diff tool integrated with Visual Studio has been improving with every version. I still prefer dedicated diff/merge tools for the job, however, and P4Merge is my favorite. This small guide describes how to properly integrate P4Merge with Visual Studio 2015.
Note that this only applies when using TFS. If you use git, Visual Studio will use the diff/merge tools configured with git.
The easiest method of installing P4Merge on your computer is with Chocolatey: