If I could pick anyone (including dead players), I'd go with Bobby Fischer. I would make Paul Keres his second, of course.
Note that I'm assuming that any dead players we would send to compete against the aliens would be in his/her prime.
However, if I had to choose from living players, I'd go with Magnus Carlsen. He is currently the world's strongest player, so why not?
Also, if I could choose from non-human players, I would go with Stockfish.



zinkelburger, check out this Google Easter egg. http://lmgtfy.com/?q=the+answer+to+life%2C+the+universe%2C+and+everything
That is a cool easter egg!