Well, for me at least it definitely is Minas Tirith. Of course, Gondolin was probably even more impressive, after passing all 7 gates, but I personally like Minas Tirith more...I actually think that Aglarond was probably a nice place...I mean Dwarves really liked it... just about any place in ME has its own beauty (ok, of course except Mordor or Wilderland)
For me it would have to be Rivendell, as it has all the natural beauty you could hope to see, and still have more conventional dwellings. Lothlorien must be stunningly beautiful, but a bit to rustic for my tastes....
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I am Anduril, who was Narsil. Let the thralls of Mordor fear me.
The most beautiful place in MD is strictly personal. I'll bring into discussion just a few points which in my opinion are relevant. The Dwarves only after a short look over their psichology, I'd say instantly that for a dwarf the most beautiful place would be a mountain area of course for which the oportunity of digging for resources and gold would be iresistable. I remember my grandmother saying that the most beautiful place on Earth is the field where you harvest for food for the simple fact that the ground is a life giver. I could never understand that. Working the ground is hard and takes long time but she was happy inspite of all conditions. Dwarves have a similar tendence of loving and pareciating their dark caves and labor inspite of all dificulties that I, simple mortal, see them impossible to love. As for an explanation for this I'd say doing the same thing for generations shapes your psichology in an unique way, particular and racial. The Hobbits are farmers mostly, small and amazing creatures as Gandalf used to say, you know them for 100 years and they still can surpize you. For a hobbit the green fields and the hills covered with grass, the orchads, the trees frowing branches towards the blue sky, the fresh air in the morning, the houses under the ground and much more would definitely build what a hobbit would name the most beautiful place. Men are very adaptable and complex. For them is dificult to say which is the most beautiful place in MD. I know Boromir and Aragorn talking about the White City being very beautiful. As for myself, Lothlorien and The Shire are indeed of a rare beauty but I'd rather not say most beautiful for I don't know all the place of MD.