Looks like you posted under the "ATI" subsection. Are you looking at overall or just on the red side of the house?
Based on pure performance shrkbay is correct (though in the case of the 6990 I think many folks would take the very slightly less powerful GTX 590 instead since it is so much less noisy).
The GTX 580 is an amazing card, and does take the cake for single gpu card while being very quiet. However, the 6970 is also quite powerful and costs substantially less (it is also faster in crossfire than two GTX 580s even though the 580s cost more). Pure performance is rarely all you want to look at unless budget doesn't matter to you.
At the end of the day there is no simple "what's the best card" answer. You should look at your price range, your monitor size/resolution, and the games you play to pick something that meets what you're looking for.