Marcus Hutter's [The Fastest and Shortest Algorithm for All Well-Defined Problems][1].

This kind of goes against the spirit of the other offerings in this list, since it is only of theoretical and not practical interest, but then again the title kind of says it all.  Perhaps it should be included as a cautionary tale for those who would look only at the asymptotic behavior of an algorithm.

  [1]: http://www.hutter1.net/ai/pfastprg.htm