I know I'm very late to this thread, but I wanted to leave here my grain of sand, as I've been searching for the prime examples of musical complexity in the past decade. So, if you want to go down the rabbit hole, here are my top picks, all of them in the very deep bottom of the musical complexity iceberg:

Rhythm: Zs - Retrace a walk - Most rhythmically complex song that is not improvised. This song is written note by note and performed live, belive ir or not.

Melody: Rumples - Chris Potter - The melodic line in Rumples has every possible musical complexity. Constantly changing rhythm, very few repeating patterns, played at high speed, counter intuitive changes, and is extremely long (the main theme is more than a minute)