This script is to be used in a Unity3D project (tested with unity 5). Just drop it anywhere in the scene and it'll cause the game's window to stay on top of the others on Windows computers (I don't know of any equivalent for mac OSX). It was put together from this script and that script, with huge help from @bzgeb & @ragekit (aka I didn't do anything myself).
The script contains useless lines and could probably be improved (eg. it currently moves the window to the top left corner of the screen on startup, that could probs be prevented??). You'll also not see anything happen in the unity editor with this script, it'll only work in the windows build.
If you identify issues with the script & know how to solve them hit me up!
/// update ///
@ragekit made an updated version of the script, that features options for borderless windows, transparency and clickthrough. Find it here.