It's hard to pick a favourite. But probably CTRL, because it's used a lot in keyboard shortcuts. Also, almost no program reacts to a CTRL without a combination key, so CTRL is usually what I use to bring a PC out of monitor standby mode.
TAB is also very nice, indenting as well as tabbing between programs (which requires AltGr+TAB here). On the topic of AltGr, with a Danish keyboard layout I need it to produce chars like @£${[]}| - located on 2347890 and key-left-of-backspace, respectively. (Remind me to start switching to US layout when programming, since those chars are mapped a lot nicer there. Yes, I am fluent with US layout as well, because of the old DOS days.)
Win key is also nice - I use Win+R a lot, as well as Win+E. Also, the context-menu WinKey is a lifesaver.
My keyboard has a Power button, I find it very convenient to avoid bending down to reach the case.
I hate those keys, all the keyboards I've seen have had them located so I hit them accidentally. I really do miss volume up/down on my vanilla IBM keyboard, though