the eVGA version is the best only on this basis:
That it can be overclocked to and past the 6800Ultra speeds fairly easily. That and eVGA is a pretty good manufacturer when it comes to making good and stable cards.
Other than that there's nothing that seperates the cards.
Unless you see one that has a lower core/memory speed for whatever reason. But you are paying for the quality of the manufacturers work. Each one will be different from the other, and without testing it yourself you'll never know which one performs better.
Which of these 6800 GT 's is the best ?
You can't really ask a question like this, as it's pretty vague. Unless you test them all you'd never know.