I would choose Mandarin, because it is the one language I studied that I was not able to learn, Turkish, Maori, Japanese, and German were more difficult for me than the romance languages, but possible. Mandarin eluded me-- although I can understand a bit-- general topic kind of understanding, I could never make myself understood AT ALL. Frustrating! I would also like to be fluent in Mandarin because many of my adult students are from China.
If not that, then sign language and braille -- because sign language looks beautiful and I would love to learn to speak without words, and braille because it would be handy to be able to read in the dark.