What is the fastest unpressurized aircraft?
Messerschmitt Me 163 Komet was unpressurized, rocket powered WWII interceptor. 


  The operational ceiling was limited by what the pilot could endure for
  several minutes while breathing oxygen from a mask, without losing
  consciousness. Pilots underwent altitude chamber training to harden
  them.


Special diets were prepared for pilots to prevent gas expansion in the intestinal tract during ascent.

The "never exceed" speed of this aircraft officially was 900 km/h but that has been exceeded anyway, when the test pilot Heini Dittmar reached 1,130 km/h. 

About 370 were built.