As far as I have found the main difference between those cards is their core clock frequency.
So as it goes:
Gigabyte GTX 670 OC - 1058 MHz (boosted), 980MHz (base)
Gigabyte GTX 670 WF2 - 1019 MHz (boosted), 941 MHz (base)
Asus GTX670 DC2 - 980 MHz (boosted), 915 MHz (base)
Which one - boosted or base - comes from the box?
And if that's the only thing that means then Gigabyte OC is the best card.