Well according to Tom's Hardware, the BEST single graphics card (for gaming) on the market is the AMD HD5970. The best single GPU graphics card is the Nvidia GTX480.
http://www.tomshardware.com/reviews/graphics-card-geforce-radeon,2761.html
So you would have to search for the manufacurer that puts out the best version of each of those cards. For AMD, I would assume the Asus Ares ROG, and for Nvidia, I'm going to say the KFA2 GTX480 LTD OC, with honorable mention to the MSI GTX480 Twin Frozr II, and the Gigabyte GTX480 Superoverclock.
Of course, even better cards are expected from both AMD and Nvidia by the end of the year.