By the Light! I was expecting some serious disagreement to this! Oh well, as I said, "That's the human-mind for you!"
Anywho, I have to say that some excellent bits of information have slipped into this thread, much of which I agree with. I too view Sauron as the "Dark-Lord" and Melkor as the "Dark Nutcase."
I view Sauron as a better villain for this reason simply, I view corruption and loss of "humanity" as a worse than beginning as the darkness. This is the differance between Melkor and Sauron. Melkor started out loathing the works of his peers, doing as he would to quell all that they made and sought for. A lust for destruction that always exists is somewhat sad, for Melkor was always the hate-stricken thing that he was.
Whereas Sauron became tainted by greed and the guilty pleasure of the agony and misfortunes of others. That he was so changed of mind to throw away all that he had and sever all of his old ties to the Valar is a much more dreadful act, for he is destroying that which he once had to pursue a path of darkness and greed. Indeed, I believe that self-corruption, and the corruption and torment of others are far worse fates and lusts than a simple and highly unlikely will for ultimate destruction.
But I must admit, I think that Melkor would most-likely be more interesting and, in my opinion, a better villain if his mind and goals were as structured as Sauron's were. Who knows? Perhaps he could have finally removed his enemies if this had been?