The one that is most powerful, has the most controls (zoom, aperture, auto mode, manual, tilt, rotate, close-up), uses the most convenient battery (AA), fastest recycle time, and at the most affordable price (if money is not a concern, then go for SB80DX, SB800, or Metz hammerheads).
Personally, I have used SB25 on my S602zoom to very good effect, both at Manual and Auto modes.