I'm thinking he's an NT because his plans are well thought out. He accounted for many contingencies, giving his opponents very few tactical options. The fact that he didn't predict the destruction of the ring doesn't make him a sensor, only stupid.
I don't think he's an NTP, since he doesn't live with his parents and he's actually executing his plan. So that would make him an NTJ.
I'm not sure if he's an extrovert or introvert. Did Tolkien write any stories about Sauron's teenage or early adult life?