Gfrktyl

Download the AlwaysOnTop installer and install it (choose the latest release). Run the app from Start -> All Programs -> AlwaysOnTop. It sits in the system tray. Click the icon in the tray, click "AlwaysOnTop", and then click the window you want to stay on top (the cursor will be the cross while waiting for the window).

Full disclosure, I am the app author and this was built using .Net (C#). Full source can be found at the link as well.

This is off the OP's original question, but seems like a good place to share information about 'always on top' utilities, even if it does require a few clicks.

From a programming standpoint, writing a simple utility like this is a very simple exercise. Maybe that's why there are so few solutions out there, or perhaps why there are so many home grown ones.

It's hard for me to trust utilities that are downloaded from somebody's Google Drive or Dropbox account. But I do trust Nir Sofer of Nirsoft. He has a tiny app called WinLister where we can set a window to be always on top. It's not very user friendly if you do it often, but I only occasionally need to pin a window to the top, so it works for me.

http://www.nirsoft.net/utils/winlister.html

I have been using VirtuaWin since many years for this. It allows me to middle click on the title-bar of any window to make it stay 'always on top'.
Works fine on win 10.
Free, open source.

You can use maComfort.

After installing, in addition to many features, you can middle mouse click on the title bar and choose the "always on top" option. You can disable it the same way.

Is there any solution that work GUI way without requiring keyboard, like long pressing/right click on title bar and select Always on Top? If you're used to Ubuntu Linux you know this functionality.

Yes, try WindowTop from here: http://windowtop.info/

from softpedia review (source: http://www.softpedia.com/get/Desktop-Enhancements/WindowTop.shtml) :

there are several functions at your disposal. These are accessed from a new menu attached to every window’s title bar. Menu options only appear on mouseover, and the container can be moved across the title bar.

It works really well on windows 10

This perhaps doesn't exactly address OP's request, but Stick-It! is a great option for keeping certain Windows on-top of others.

Key features:

    - Portable

    - Works with Windows 10 (tested, even though the landing page for the software doesn't

      state as such)

    - System tray controls

    - Doesn't retain on-top window settings through log out/restart/shut down

Windows are selected to keep on-top by right clicking on the system tray icon the software sets up, selecting "Stick-It!", and then simply clicking anywhere in the window of choice, which wiggles the window side to side for a second or two.

You can use AutoHotkey and make a script with just a single line of code:

^SPACE:: Winset, AlwaysOnTop, , A

Ctrl+Space Will toggle a window to stay on top.

Have a look at the full syntax:

WinSet, AlwaysOnTop , OnOffToggle, WinTitle, WinText, ExcludeTitle,
ExcludeText

A activates WinTitle A which means the Active Window.

Read more on Winset command here:
https://autohotkey.com/docs/commands/WinSet.htm

If you're unfamiliar with AutoHotkey, download it from here:
https://www.autohotkey.com/

And learn to use it:
https://autohotkey.com/docs/Tutorial.htm

Explaining AutoHotkey would not be relative to this answer, and the tutorial link will explain much better than I can.