I assume you are talking about a softbox that attaches to a hotshoe type flash?
There are many, ,many flash accessories, and most of them have their strengths and weaknesses...they work better in some situations than in others.
When we talk about the 'softness' of a single light source...there are two factors. The size of the light source and the distance to the subject. This is where a softbox is good, because it's typically a good bit larger than the flash head, which softens the light. The 'Fong Dong' and especially the Omnibounce do not significantly increase the size of the light source...so by themselves, they don't significantly soften the light.
One of the best methods for softening the light, is to bounce it off of a large surface like the ceiling. This can give your subject a nice soft light that comes from above, which tends to look fairly natural. The problem then is that the light from above may cause shadows on their face, just like mid day sun. This is where a lot of flash accessories come in. They split the light so that some goes up to bounce and some goes directly to the subject to fill in shadows. A simple bounce card can do the same thing.
The Lightsphere sends light out in all directions and if it hits something and comes back, it can give you a really soft light. But if there is nothing to bounce the light off of...it's wasted. Good for small rooms, not so good in larger rooms and practically useless outdoors.
There really isn't a 'best solution' for flash accessories. The right one for you will depend on your style. Better than a flash accessory is knowing how to use your flash effectively.