The most beautiful language is Hebrew, especially in singing.
The hardest language would have to be any of the un-written tribal languages of southern America, Africa or Asia, because the only way to learn them is to be part of the tribe! You can't learn them on ITALKI, or in any University! Another contender for the hardest language would have to be Ancient Egyptian (Heiroglyphics).
The most popular language must be English, because it has become the language of the world (somehow!)
And finally, the language I like best is Hebrew, because it will be the single world language of the Messiah's reign!
"For then I will restore to the peoples a pure language, That they all may call on the name of the Lord, To serve Him with one accord." The Tanakh (Bible), Zephaniah 3:9