There are plenty of different flash accesories...they each work better or worse in different scenarios. The 'Fong Dong' for example, spreads light in all directions, with the intention that it will bounce off of walls and ceiling, thus giving you are light that surrounds your subject. That's all well and good if you have walls and a ceiling that are nice and close for each shot. If not, the light is just sent out, which means wasted light...which means faster battery consumption and slower recharge times.
Many of the accessories are meant to help when you are bouncing the light off of a ceiling (or wall)...and they throw some light forward as fill. The Omnibounce, is an example of this. It's meant to be used with the flash at an angle so that you are bouncing most of it off of something...but it also throws some forward to fill in shadows that the bounced light won't get (also helps for catch lights).
A simple bounce card will do exactly the same thing.
I often see photographers put the omnibouce on the flash and fire it directly at the subjects...thinking that the opaque plastic will make the light softer. This doesn't really work because the softness of the light is determined by the relative size of the light source compared to the subject...and the omnibounce isn't really much bigger than the flash head...so it's not really softer. It just wastes more light/power.
You can get a softbox for the flash, which does increase the size of the source...which does make the light softer. These can be a bit of a pain to work with though.
You also need to consider your camera orientation. A lot of shooting is done with the camera in vertical/portrait orientation, which puts the flash off to the side...this can create some bad looking shadows on things behind your subject. This is where you want a flash bracket to keep the flash above and in-line with the lens.
There is no 'best' accessory or even the right one...you just need to find what works for you...but understanding light will go much, much farther than any accessory.