Fastest would be the VelociRaptors from WD. Then it would be the WD Caviar Blacks. The Velociraptors dont offer much price/GB value. Because of that the Caviar Blacks are the best. I am not sure if they are available in 500GB. But you can check the website for it.
true but there are some kick-ass other hdd's:
SAMSUNG Spinpoint F3 HD502HJ 500GB 7200 RPM 16MB
Once again my Barracudas challenge you Spinpoints.
The velociraptors may not have much more than the Spinpoints or Barracudas on disk transfer, however they have massively better seek times making then much better for dealing with small files.