Hardest song you ever played in your entire life and why it is too hard?

In terms of right hand technique, Meshuggah - Bleed cover really pushed me to my limits. In order to sync with guitars and drums I used all 4 fingers on my right hand.
In terms of left hand technique, my band Achroma Rising's song "Water Devours The Earth" (soon to be released) was very challenging to record. I wrote all the bass parts on the computer first, without ever touching the instrument. When I finally picked up my bass for recording, I realized that the bass solos in the song required extremely uncomfortable left hands positions, as you can see in the video below. But overall I am glad it turned out that way, I feel that my playing improved vastly while recording that song.
Also, I am writing some very crazy, out-of-this-world-difficult bass parts for the other upcoming Achroma Rising songs. I am not sure how am I going to play those, but I will figure it out :)

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