The Fairest Kingdom/Realm in Middle-Earth

[Gurthang]

I would have to say that as far as the specifications you mentioned, I agree with Formendacil.

Beauty -- It might go unnoticed, but there is a large lake in Mordor, and I've heard tale that it is surrounded by a fair country that's good for farming. Sauron must feed his armies, after all.

Mechanism -- Mordor certainly had many of those, although I'm not sure exactly what kind. They would have to be substantial though, because Sauron used very large seige engines and catapults in his attack on Gondor.

Efficiency against its enemies -- Besides the huge armies to guard it, what better to protect your country than impassible mountains?

But I should point out that Lorien would not be far behind. They would only lack the Mechanisms category to overtake Mordor. I actually think that Lothlorien surpasses Mordor in the other two respects.