I use all the keys listed, more or less, except the "Mac" key as my IBM laptop lacks one. I voted for "F12" before i remembered that that, when combined w/ "Shift", is "fvwm reload" key.
Given the miserable choices, I should have actually voted for the "Pause|Break", which i haven't used much in a while. On the third thought, vote should have gone to "Mac" key (as i did press "Pause|Break" w/ in last 5-6 years to cause FreeBSD kernel debugger to load (for kicks of course)).
I am highly surprised that there is no "NumLock" option; that's a rather useless key on a small|cramped keyboard (where numbers are overlayed on the staggered letters).