200 Great Classical Music Works: https://www.listchallenges.com/200-great-classical-music-works
This list does not contain all operas ever written, but more than most other guides. I hope it can inspire us to new discoveries.
The list also contains some important operettas as well as some examples of other genres often performed on stage (cantatas etc.).
Most titles are given in the original language, the Russian (cyrillic) ones are translated. Sometimes the picture contains the English title, which may help you.
Check only if you have heard (seen) the complete opera (live in theatre or on record/radio/video - anything goes).
Do not be disappointed if you haven't found something here. In some months I plan to make an extended version of this list, which will contain more rare works for true opera fans.