Spanish does it for me. Then Brazilian Portuguese. Whether you understand them or not it they fall softly on the ear.
I don’t know, Nekochan, the onomatapoetics are my least favorite part of Japanese. Sure, they come in handy now and then, but when the news anchor is using cute little sound words to describe how the murderer chopped his victim into twenty pieces (barabara satsujin), it gets a little ridiculous.
Personally, I like Japanese (despite the sound words) and Mandarin the best, but German is particularly nice when you want to sound eeevil.
–sublight.