Too early to tell, IMO. Wisconsin and Arizona are both great teams, but neither has been dominant so far. Arizona, in particular, I'm not yet sold on. They have surpluses of size up front, but they don't really have anybody they can consistently turn to for a bucket late in games. In this sport, you really need a guard who can score. I'm not sure they have that.
Kentucky and Louisville have both been incredible defensively, but neither of them have shown me enough on the other end for me to call them dominant quite yet.
I still don't think we know how good Duke is defensively. They surrendered a lot of easy baskets to a pedestrian Michigan State offense. I think we'll learn a lot about Duke when they play us on the 18th.
Don't discount UNC or Villanova. Gonzaga looks good, too, but we all know their track record - they'll have to prove that they can get out of their own way in March before we take them too seriously.