When Windows 95 first shipped, it came with a readme file that outlined the use of the Windows logo key, which was first included on standard PC keyboards at that time, and has been included on all since. As far as I know that file has never been included with any subsequent release, though the function of the key has not changed in all that time.
Win-M to Minimise all
Win-R to Run
Win-L to lock screen
Win-E to open File Explorer
Win-Up arrow to maximise window
Win-Down arrow to restore
There are a couple of others. These are my "favourite" shortcut keys, but if you're asking which I use most often, it's Ctrl-C and Ctrl-V, and all others are a distant second.