This is the most beautiful thing anyone has ever seen.

It is a little odd. It may be more accurate to say "this is the most beautiful thing I have ever seen" because it is impossible to tell what everyone has seen and to know that what you are seeing is more beautiful that any of that! That being said, some people do say in English "that is the most beautiful thing ever" which implies that nothing could possibly surpass it in beauty. :)

This is not correct.

This is the most beautiful thing anyone haven't seen