What does 没有最好只有更好 mean?
意大利文化 // 真的是 // 没有最好 // 只有更好的 
  Italian culture really doesn't have the best (thing), only better (things). 


The 《没有最好，只有更好的》 is the quality linked to by the copula 是。

With some more thought you could surely find a more fluent translation.

Anyway, it's an elegant way of saying that Italian fine arts and cultural heritages are so many (common, widespread) that you can't just single out THE best one, but they all rival each other.