I have just installed Unity 5.5.0f3 and it is not working properly. Unity and Visual Studios installed ok, and launching Unity seems to work fine but when I edit a script it opens Monodevelop that throws an error, checking the settings it is pointed at Visual Studios it is still opening Monodevelop. can you recommend a fix, I have tried reinstalling the entirety of Unity and visual studios and it still does not load Visual Studios on double clicking a script. Also MonoDevelop was throwing an error in reference to project files, but upon trying it again just now it did not throw an error and everything seemed to work ok, so I changed the external tools for editing inside of Unity (as it was pointed as VS Express.exe directly) back to Visual Studios 2015 and reloaded Unity and it still opened Monodevelop but with no Syntax Highlighting or Auto-Complete.
Is the default program for opening scripts set to VS in your operating system?
Small update, Been a while since I’ve been able to even look at Unity in a while. So busy with other stuff.
Just went and downloaded MS Visual Studios 2017 fresh from MS Site. Inside the installer there is a Unity package you can select, so I selected and installed it.
After installation ran the VS 2017 for Unity Package (With Unity open) and that added a few files into the Project files). Then created a shortcut for MS VS 2017 on desktop and pointed Unity at it for the External Script Editor Tool and upon double clicking a script it opened VS 2017 and set up the project for it. 
Happy it seems to work. Will keep an Eye on it.
Not sure why the 2015 didn’t work, I’m thinking it either didn’t install properly, twice, or I missed something (maybe I didn’t run the Unity Package for VS 2015, can’t remember now).