Not a fan of Gilbert and Sullivan at all though I have seen PIRATES, MIKADO, and will be seeing a friend in PINAFORE a week from Friday.
The only operetta from the golden age of them I've ever seen on stage was THE STUDENT PRINCE which I liked very much. If KISMET is an operetta (and I can see the case made for it), that's definitely my favorite.
I really like the movie versions of operettas like THE STUDENT PRINCE, THE MERRY WIDOW, THE DESERT SONG, THE VAGABOND KING, ROSE MARIE, MAYTIME, etc.
And I truly enjoyed that series of Decca Broadway reissues of operettas they recorded in the 40s with their studio contract stars like Kitty Carlisle. Very, very entertaining, and the music is beautiful.