今までで見た中で一番きれいって英語でなんて言うの？

I would suggest always using a '!' when saying something with such passion. Wow! Amazing! That is gorgeous! If you said such a phrase without emotion you'd lose the effect of what you are saying. Both of these phrases will work for beautiful scenery or anything you are wowed by!

Well, if you wish the conversation to be about you, then you could use the first sentence. However, if you wish to make an enthusiastic comment about the exquisite beauty of a landscape or building or work of art, then the second example may be more expressive and appropriate.

When trying to describe a most beautiful sight there are a variety of adjectives that one can use to express him or herself such as ;
magnificent- extremely beautiful and impressive to the eye.
glorious- having immense beauty and striking.
majestic-showing impressive beauty.

"I have never seen something as beautiful as this" would be an acceptable general term - whether referring to a sight or anything. Words like "wow!" really emphasize how impressive the sight is. "This is the most beautiful place I have ever seen" would also be appropriate. Remember to give reasons why you think it's the most beautiful sight you've ever seen.

If you want to describe something as being very pretty/beautiful then you can use the words 'beautiful, wonderful. magnificent or glorious' they all mean something is very pretty
To add 'I have ever seen' means you have never seen anything as beautiful/pretty as it before

The two sentences you see provided above are terrific ways to express to your listener that a particular sight was the most beautiful sight you have ever seen. In the first sentence you will notice the adjective breathtaking and in the second sentence you will notice the adjective gorgeous. Both of these words mean beautiful. These two words are common in our everyday conversation and they would make great additions to your vocabulary.

When you want to express that what you are seeing is the most beautiful thing you have ever seen before you can say, "I have never seen something so beautiful before."