"This is my favorite song."
Translation:To moja ulubiona piosenka.
Yes, it's rather not even treated as a diminutive. The word "pieśń" is of high register, with quite a big dose of pathos. Like "pieśń patriotyczna" (patriotic song) or "pieśń religijna" (religious song, or maybe rather religious hymn).
I am a Dutchman i will type "To jest moja ulubiona piosenka" is that incorrect ??.