Central Asia is quite underrated from tourist point of view but each country has something to offer to the foreign visitors. I can’t say which country of the ones you mentioned is best, because all of them have different pace of development, economic situation, and tourist attractions.
So if you want to enjoy natural beauty of Central Asia, I would suggest you to go Tajikistan or Kyrgyzstan. In Tajikistan, you will find more wilderness and many natural, sublime spots. Kyrgyzstan, on the other hand, will provide you a city life to enjoy as well, along with the natural beauty.
Loading image... (Courtesy: Heart My Backpack)
Central Asia is also famous for its Silk Road culture, and to enjoy that Turkmenistan or Uzbekistan are the best. You will find antique mausoleums, and aroma of rich spices all around.
You can also look for the following attractions in these countries. They are worth paying a visit to.
1. Almaty, Kazakhstan
Loading image... (Courtesy: Indy Guide)
2. Gonur Tepe, Turkmenistan
Loading image... (Courtesy: PicoShots)
3. Bukhara, Uzbekistan
Loading image... (Courtesy: Advantour)
4. Pamir Highway, Tajikistan
Loading image... (Courtesy: The Times)