Outdoors? On a speedlight, a diffuser is mostly a waste of flash output...it gains you basically...nothing!!! (And I mean that seriously.) Indoors, the Sto-Fen Omni Bounce at times, or the Nikon-supplied diffuser cap, the Sto-Fen clone, that came with the SB-800, usually with the flash aimed upward at the 60 Degree tilt position, sort of a
half bounce/half directed forward through the diffuser kind of deal but ONLY on shots under 15 feet!!! Indoors, for the most part, I prefer bouncing the flash off the ceiling, a corner of the room, or a wall.
Outdoors, for close-in stuff, my new toy is the Westcott 43-inch silvered umbrella, with the double-telescoping handle. It folds down to only 15 inches in overall length, and it VERY efficient.
Westcott 43 inch Soft Silver Collapsible Umbrella 2002
However, outdoors, again, a diffuser is often not all that needed when the flash is being used for
FILL-IN lighting, so...a bare, undiffused flash, Vivitar 285HV with Quantum Battery 1, or SB-800, or Speedotron light head firing at 100 to 200 Watt-seconds is what makes the most sense for me.