I am surprised nobody has recommend the two-volume series The Chauvenet Papers printed by the MAA.

The work is a collection of prize-winning expository papers on mid-to-advanced undergraduate topics in mathematics; I have read most of them myself and would say they are invaluable surveys into various topics of mathematics (examples include broad areas such as harmonic analysis or the history between mathematics and logic, problem-specific articles such as one on Hilbert's 10th problem, and more miscellaneous works like Zalcman's "Real proofs of complex theorems (and vice-versa).")