I'm not as familiar with Strauss' later operas as I would like, so I don't mean my choice to be definitive whatsoever, but I voted for Zarathustra, Salome, Elektra, Metamorphosen and the Five Last Songs. I think my favourite out of all of those would be Salome: an absolute firecracker of an opera, condensed but astonishingly powerful. Everyone who is a fan of opera should watch this performance: