Well, de gustibus non diputandum est.
I was asked to answer such a questions at Quora.com. So, challenge accepted!
However, trying to judge beauty of a certain language one should consider at least two things. At first, how does it sound? – Is it nice for ears? And secondly, how does it work – how is it bulit, is there a harmony in grammar, etc.
Despite so many consonants and consonant clusters Polish may sound suprisingly soft and pleasantly. Here’s Edyta Górniak’s performance on Eurovision Song Contest in 1994.
Comparing to other European languages Polish grammar is quite complex, but it has a lot of regularities. You just need to remember a bunch of rules. There are some parts of a harmony, like a gender agreement between a pronoun, noun and adjective.
Ta kobieta jest dobra.
Grammar is like maths. A lot of formulas makes it all fascinating and beautiful in the same time.