Of the alternatives in the poll, a night session at Arthur Ashe with the crowd going crazy, for me that's special. USO isn't the most well organized slam (only RG is arguably worse), but there's something about the atmosphere there.
Other than that, two things that I always enjoy watching:
1) A big upset. Out of nowhere, someone plays out of his mind and takes down a top seed. That sort of defines what sport should be about in my opinion. The belief that everything is possible.
2) A powerhitter painting the lines. Be it Soderling, Del Potro, Wawrinka, Thiem or in the future Sinner (knock on wood)... watching an in-form powerhitter crush the ball can be such a feast for the eyes.