Um, definitely Griffin.
Butler and Cousins are just as good, Griffin is the best fit.
Butler would be interesting, that might be the longest/most athletic 2/3 duo since Wade/LeBron. If they had a defensive scheme that forces cross-court passes, they could probably pull off a lethal defense similar to those MIA teams. Also, Butler can easily guard whichever opposing SG/SF is stronger/better, allowing Durant to save energy for offense.
Gotta love Durant and Griffin in the pick and roll, though. I think Durant would prefer Griffin to DMC because he's a better/more willing passer.