Franz Liszt's Transcendental Etudes are flat-out some of the most technically difficult piano pieces ever written. To competently play these pieces, a pianist basically needs virtuoso-level skill. That said, the Transcendental Etudes also accomplish something that a lot of overly-technical music doesn't: they're great songs. They have catchy melodies and motifs, dynamic transitions, and interesting build-ups and break-downs. While purely skills-based music has its place, I am always very impressed when a composer can create a piece of extreme technical difficulty that is still accessible to listeners who don't play the instrument or instruments in use.

As far as technical skill on a piano, you're not going to find many people who can out-class Georges Cziffra. Simply put, he did everything perfectly. What impresses me most about this man, though, is his ability to make the piano sound like a force of nature. While the piano is an instrument of great dynamic and tonal range, Cziffra's skill can make it sound like a the most violent thunder-storm you can imagine or a leaf falling from a tree on a windless day. He could just produce sounds that most other people can't.

Below is a link to my favorite of the Transcendental Etudes. It's Etude No. 6 in G Minor. I recommend that you pay special attention to the bass buildup that starts to happen around 2:52.