8. Complex Villain
Whilst I admit that both chief villains in their respective franchises cannot be described as anything but incurably evil in every way, and they are both genuine villains in their own right, I feel that there is a greater deal of depth to Voldemort, that doesn't exist in Sauron. Voldemort is everything Sauron is, yet much more complex. In fact, in Harry Potter, almost one entire novel is dedicated to the life of its villain (that being "The Half Blood Prince) and we witness what drove Voldemort to his crimes and we see him develop from the young Tom Riddle into the monster that he has become. What makes Voldemort such a convincing villain is that if you shed away his monstrous exterior, he is indefinitely human.
Such is the complexity of Voldemort's character that he has managed to transform himself from human to a serpentine creature. The same can't be said of Sauron who is a demi-god and his existence is never explained by Tolkien, so naturally his character cannot be explored in anywhere the same detail as Voldemort's. Moreover, Voldemort has much more of a devoted following on his side than Sauron. He has the full support of a whole group of human villains (Death Eaters) that do his bidding, yet Sauron simply has the "support" of a horde of Orcs. So clearly, such is the influence and presence of Voldemort that he can command the support of many more people than Sauron. The final nail in the coffin for Sauron is that Voldemort in the Potter series seems incessantly evil through his sheer enjoyment of torturing and murdering others. He killed his own father, grandfather and uncle without showing a fragment of remorse and in fact he enjoyed it. Whilst this is may be true of Sauron as well, it is not made clear in the novels or the films by Tolkien that that is the case. Oh come on, how can you argue that Sauron is the superior villain? Voldemort is and always will be, the true Dark Lord.