Actually I think the 6.67 or 6.50 drivers are the fastest. I have tried so many drivers it isn't funny. Give them a try and see what you like. Some systems like different drivers better so no one driver is the best.
I've had good luck with the 12.40 drivers and get the best 3Dmark scores, but they work best with DirectX8. However, I have not tried anything above 12.40 yet. However, like Bender says, what works best on your system might be different.
I didn't have good results with the 11 series, but I'm using 12.60 now, and I get better results with them than I did with 12.41...plus, since 12.41 are certified, even with NVmax or AGPTweak, coolbits are inaccessible, but they work fine on 12.60