I use Ctrl+`. It's quite convenient as it does not require looking at the keyboard (I could not do e.g. Win + function or Pause like that) and still does not interfere with my more frequently used shortcuts (I use it a lot, but not as often as my "work" cuts).
I might be a bit obsessive about it, but I don't like to move my fingers from where they belong, i.e. the home keys (yes, I know, the sentence just invites all those NSFW comments
). As my work requires quite a lot of typing - I am a translator - I prefer to have everything close to my fingertips. In my work applications I have most of the commands available as Left Alt + home-or-near-home keys, i.e. JKL;UIOPNM (<- that was easy to type
). At some point I had even arrow keys redefined as Ctrl+JKL: but it turned out they were not used that much.
Sorry for that aside, but I think it's still on the topic, considering that FARR is all about eliminating that productivity-killing monster i.e. mouse + desktop, menus etc.