I asked Wagneropera.net's followers on Twitter what they would recommend a Wagner virgin to listen to.
This is what they recommended:
Lohengrin Act 1 prelude
Walküre: Ride of the Valkyries.
Siegfried's Funeral March
Parsifal: Good Friday Music
The Flying Dutchman
What would YOU recommend to a person who wanted to try listening to Wagner for the first time?