Nautilus has an article[1] that explores exactly this question. I beleive the conclusion was that it is the number of cortical neurons, not the total number of neurons and not brain mass that best predicts intelligence, or at least best correlates with our intuitive ranking of intelligence that puts humans and then primates at the top, above elephants.

[1] - http://nautil.us/issue/35/boundaries/the-paradox-of-the-elep...