This isn't an official nVidia thing, but i have to agree with Uggorthaholy only with a slight change.
"The ASUS Mars II would like to have a word with you."
It is not of nVidia design, and as much as people would like it not to be the HD6990 is slightly faster then the GTX590.
So if you're referring to reference cards then yes, AMD has the strongest single card.
If not, the ASUS Mars II wants to kick your teeth in for insulting it.