according to Toms Hierarchy chart
http://www.tomshardware.com/reviews/gaming-performance-radeon-geforce,3018-7.html
the HD 6990 and the GTX 590
but techically they are double GPUs on one card
so the best single GPU would be
GTX 580 and the HD 5970 (unless that is a double GPU-not sure)
in which case you would go with the HD 6970
that is going by raw performance
not using factors of heat noise power usage and price
As you can see I respect Nvidia and AMD/ATI equally
so I will not pick between them
they each have their strong points
I think though the HD 6990 currently holds most powerful GPU title
but that could be debated