Waltz of the Flowers by Tchaikovsky
What is the hardest piece you have learned?

Depends on a few things, but probably 'The Lark' by Glinka arr. Balakirev. It's an ongoing process honestly. I love to play fugues as well which by their very nature are quite difficult.

@SoccerSucker Yeah, it is a bit. The more you look at it though, the more you realise that it's hard, but not impossible. The cadenzas sound half decent 3/4 slower than most people play it and they often have a pattern that you just have to figure out. The runs are nearly D Flat major scales, a nice key for fast and light movement... I'm pretty lazy though, so hence the ongoing process. If I knuckle down, I'm sure it would only take me a couple of weeks. Don't underestimate the power of motivation and determination.
Mine is Fountain in the Rain by William Gillock