From my Collections (all from Youtube and the Internet Archive, except films)

Operettas I have seen: Everything by Gilbert and Sullivan, The Merry Widow, Perchance to Dream, Orpheus in the Underworld, La Belle Hellene, King's Rhapsody, Die Fledermaus, and Merrie England.
I also saw the film version of glamorous Night, which was a disaster other than Mary Ellis and Trefor Jones, and The 1950 version of The Dancing Years, which I can't recommend highly enough, as it was perfect.)

Operettas Left to Watch: Salad Days (liked the music), but haven't seen it), The New Moon, The Student Prince, Tom Jones, Firefly (radio production), La Périchole, and The Red Mill (radio production).

Operettas Only in Music (need full versions): Bitter Sweet, Countess Maritza, The Chocolate Soldier, The Land of Smiles, everything by Novello and Lehar not mentioned above, plus any other operettas in English. Please keep suggestions within the old style, not modern or modernised.