I think that the hardest piece is the one where the emotional of the music can be conveyed. Yes technical skill takes years to build but bring out the heart and soul of the composer’s intentions is where the real talent lies
The most technically difficult pieces are Paganini caprices 1-12. The most hard pieces to play musically would be any of the fugues from Bach’s violin solo sonatas and partitas. I also think that Wieniawski concerto 1 is one of the hardest pieces to play because its a marathon the whole time!