This is an interesting question. Here are my answers.
Here are the three best in my opinion:
1. Spanish--One of the best sounding languages ever which I knew since I was very little. I don't get why everyone puts it on their bottom three. In my opinion it is a beautiful language, the language of love.
2. Russian--I'm learning it now on Rosetta Stone. Their accents are awesome too.
3. Turkish (I like it's vowel harmony and the way it sounds in general. That's why I based my conlang Yampa off of it)
The three worst:
1. Korean--way to many vowels, too sing-song for me. It annoys me to death when all the Korean girls speak it in my school (I'm sorry if that comes across as racist but that is the truth)
2. Arabic--Just the opposite of Korean, a very guttural language with a lot of hard consonants.
3. German--Everyone sounds so angry when they speak it. Even "I love you" sounds like "I want to kill you."
Again, I'm not being racist. I'm just purely talking about language here, not peoples cultures, races, ethnicities or religions.
