ReSharper : Resharper is at the top of my list. It can be challenging to use low end computers but if you have considerably powerful machine you won't regret to use it. It finds your code smells, suggesting better naming suggestions, finds more specific warnings and problems that even visual studio itself not able to detect and automates your coding routines. https://www.jetbrains.com/resharper/
GitHub Extension for Visual Studio : Best way to connect to your GitHub repositories and manage them in Visual Studio easily.
Bitbucket Extension for Visual Studio : If you want to manage your private repositories in Bitbucket and looking for an extension to easily do that there is no official extension about it but that extension is really makes it easier to manage your Bitbucket repositories. It's seems safe and getting updates frequently.
Power Commands for Visual Studio: Power Commands adds small functionalities that improve your user experience in Visual Studio. This extension is primarily around navigating the IDE and easily find your way in menus.
File Icons: Adds icons for files that are not recognized by Solution Explorer
Image Optimizer: Sometimes i forgot to optimize the images that i upload to my web apps. Generally i'm using some online tools to optimize images. Now I just right click an image inside Visual Studio and easily optimize it.
VSColorOutput: There are too many information pops up when you did something about your project in visual studio. That extension highlighting most important ones for you https://marketplace.visualstudio.com/items?itemName=MikeWard-AnnArbor.VSColorOutput
Glyphfriend: Sometimes really hard to remember a Glyph icon. It shows all icons in a dropdown when you are typing.