As a guitar player, I am probably biased in saying “The Electric Triangle”...
Just joking. In my opinion, it would be the electric guitar. The guitar is an exceptionally versatile instrument, and unlike the piano, has a ton of crazy technique which can be used for better play-ability like sweep picking, or fret tapping for example. Plus, in the genres of rock and metal, the guitar is pushed to it’s limits and it’s mindboggling what masters like Vai, Malmsteen, and others are capable of. For example:
Angel Vivaldi – https://www.youtube.com/watch?v=qFWoIyqSjlA
Richie Faulkner – https://www.youtube.com/watch?v=45Bh1Hkdf5Q
Steve Vai – https://www.youtube.com/watch?v=aK532U3emOQ