I would rather say , "He enjoys the beautiful nature of Siberia."

Some options for wording this sentence: He enjoys the natural beauty of Siberia. (Modt simple/concise option) He enjoys Siberia's natural beauty. He enjoys the beautiful natural landscape of Siberia. He enjoys the beauty of Siberia's natural landscape. In this case, 'beauty' is used in singular as in this kind of description it's uncountable. You can also use nature--> natural as an adjective or beauty-->beautiful as an adjective.

the second one - THE - would work the best out of these two sentences.

Both sentences are grammatically correct, though their meanings are ever so slightly different. I would say "He enjoys the beauty of nature in Siberia". You might need to use "beauty" instead of "beauties". I don't know which is right for you because it depends what you are trying to say. Both are correct but they have different meanings, which I will now explain. When you say "beauties" you are talking about specific special things that stand out in your mind, like some flower that you are especially fond of or some special place. To make a comparison, have you heard of the "wonders of the ancient world"? There are said to be seven of them: the Hanging Gardens of Babylon, The Pyramid of Giza, and so forth. The phrase "beauties of nature" is similar to the phrase "wonders of the world". Here is a sentence using "beauties of nature" correctly: "He enjoys the beauties of nature in Siberia, such as Lake Baikal, the Altai Mountains, and the Chara Sands." It is not necessary that you list the beauties, but you shouldn't use the word "beauties" unless you have such a list in mind.