The Tax Code?
This is disturbing. Of all the possible choices, you’d consider the TAX CODE? That’s just not right. Forget about autism, knowing the entire tax code would have to induce some sort of mental illness – probably schizophrenia.
The tax code. Yeesh!
Mandarin, because my wife’s side of the family doesn’t speak English. A scientific calculator would be useful too. I’d give up my memories from birth through middle school if you could toss that in as well.
Not exactly an extraordinary wish, but I’d go with Google maps. I’d never have to look up directions again.
A calulator brain. So I could find, say, the value of tan[arcsec(7/23)] in my head.
PDR (Physician’s Desk Reference) or Harrison’s Textbook of Medicine or Rosen’s Textbook of Emergency Medicine.
I’d go for savoir faire: knowing all the wines, the right thing to wear, the gentlemanly thing to say.
Second in line is the Brazilian Jujitsu.
I don’t know if it would really be the thing I’d use for my one download…but it is something I’ve often wished would be fed to me Matrix-style:
The perfect golf swing.
It might make the game too easy and boring, but I’d get my joy out of blowing people’s minds.
If we can only have one complete language in the upload, I’d pick the ability to learn languages easily instead.
That way I could just cruise into a bookshop, grab the “Russian in 10 easy lessons” set, absorb it, watch some Russian TV, listen to Russian music, start IMing in Russian, start chatting with neighbours in Russian, etc… and just pick up the language. But in a couple of weeks instead of five to ten years.
Barring that, I pick French for my language upload. 
Oh, Sean Factotum, I <3 you. So selfless, sacrificing yourself for your fellow man. 
I’d upload some pattern drafting software, that would allow me to draft patterns for anything from bliauts to Elizabethan doublets to Victorian corsets to frock coats to . . . . ANYTHING! Double happiness points if I can do it just by looking at someone.
Barring that, something infinitely more esoteric: the ability to read and speak Old English.
jawdirk’s suggestion of savoir-faire is pretty tempting, but I’m going to have to one-up the kung-fu guys and go with gun-fu. The reflexes and grace to pull it off (even at “mild proficiency”) would have to be ungodly, and you could use them in a reasonable number of other physical skills.
Yeah, so I’d probably never use it outside of an Airsoft range. That’s okay; it’s fun anyhow.
I’ll go with a better sleep function.
As it is now, sometimes I lie awake in bed and can’t get to sleep, or wake up in the middle of the night and can’t get back to sleep.
I want to be able to go to bed, fall asleep immediately, and stay asleep until it’s time to wake up.
I’d love to be able to close my eyes and play Tetris.