Besides my mind.
A former mother-in-law of mine, whom I was quite fond of btw (imagine that) had a little plaque on the wall next to her kitchen sink, which read "Of all the things I've lost, I miss my Mind the Most".
What reminded me of this is that I just picked up the TV remote.
The day I brought the new wide screen TV home and set it up, inside of the first hour of deployment, I lost the little plastic cover that conceals the two double AA batteries.
(redundant I know, you won't like the BAR complaints department).
It has not turned up to this day, and every time I manage to find the remote, even after all this time, a faint "DOH!" still bounces off the back of my brain pan.