I had to vote for "SysRq," since not only have I never pressed it, but I've never had any idea what it was for, ever since I first saw one in 1987. Although, I do use "Print Scrn," which is on the same physical key, to take screenshots under Windows pretty often.
I'm surprised that nobody has mentioned the "Application" key (the one with a little picture of a drop-down menu and a cursor) on Windows keyboards. Surely that is more useless than the Windows logo key, which allows Win+R, Win+L ("Lock Workstation" on Windows XP), etc. I know it's supposed to bring up a context menu, but it's easier for me to just press Shift+F10. I guess I could remap it to something else, like I already did with Ctrl/Caps Lock and Esc/~ for Vim.