in reply to The most useful key on my keyboard is:
Most Useful = Most Powerful, no?
Hence Ctrl is the most useful since it's the root for all hot-keys. (windows Alt based not withstanding - and many have Ctrl equivalents now) I'm going to go a step further & point out that the CMD key is missing from the list - which is the most useful key on a Mac. However almost all of those hot-keys have their equivalents mapped to Ctrl on other platforms - and since those platforms vastly outnumber the overall number of macs (at least outside my house) then clearly Ctrl is the hands down winner. And by "hands down" I mean don't touch that mouse since it can only slow you down.
Then again - if I were talking about using the vi editor than the most useful key is the ':' key.
Wait! This isn't a Parachute, this is a Backpack!