Opera in the Twentieth Century

It's all Mark's fault! Ionarts contributor Mark Barry wrote about his experience with Hurricane Alex last week (cited on Artblog: thanks to anyone who has come here from there!). Well, now that Mark's back home, he appears to be some sort of hurricane magnet: the remains of Tropical Storm Bonnie drenched Washington with rain last night, and Hurricane Charley will apparently follow, once he has done far worse damage in Florida.

I will be teaching a course on opera in the twentieth century this fall, and I have drawn up my syllabus. Here is the question for all you opera fans out there. What twentieth-century operas does one simply have to cover in a course like this? What are the operas that were the most significant in the last century? We are working on the following assumptions, that the work was premiered in the 1900s, that it is entirely sung, and that it was actually staged. Please use the Comments section if you have any thoughts, and I will see how my views match up with yours.