The following question has been inspired by "Must read" papers in numerical analysis.
In 1993, Prof. Trefethen published a NA-net posting with a list of thirteen papers that he had used in the seminar Classic Papers in Numerical Analysis.
Question: In your opinion, what classic papers should be in a "must read list" of papers in the [huge] field of PDE (and related areas)?
Note: Paraphrasing Trefethen, let us intend "must read" as providing a satisfying vison of the broad scope of PDE and a sense of excitement at what a diversity of beautiful and powerful ideas have been invented in this field.