Mate in 3
There is a lot going on it this classic chess problem by Anderson and Eaton, 1953. It has a theme (prevention of black checks on a5, d8 and g5, sometimes by black preventing it himself) and a wonderful variety of different mates.
In the solution I have given 13 different first move replies by black and every one of these must be met by a different white 2nd move, with several different mates arriving on move 3. This a monumental work of chess construction!
The key is not easy to find but the variations are delightful.