Very difficult question to answer.
The key to U2 musically is their ability to take something simple and make it profound.
I looked over the entire cataloge and couldn't think of a single tune that was "musically" complex. (i.e. difficult chord changes, odd time signatures, etc.)
I guess I would say the most musically interesting thing is Edge's guitar sound. You know, his signature ringing sound. "Pride" "Streets" "Wowy" etc.
Bono summed it up perfectly saying "Edge owns that sound". It's true. In the same way Hendrix before him, Edge came up with, or at least popularized, a sound that was unique.
When I listen "Heartland" for example, I can't think of a more perfect sound to fit the theme of that tune.