The coolest, most action-packed, exciting sporting event I've been too live, and its not even close, was a US Open Semi Final where I saw Federer Play Djokavic in a 5 set epic batter than was the start of Novak's dominant run, and then same day saw Nadal in his prime destroy I can't recall who. You really get an appreciation for much of a battle the sport is.
The best time to go to the US open is the first week.. You can get some marquee names on the smaller courts where you can really see the skill these guys possess up close. I went to the US open semi-final and final, the year the Del Portro beat Federer and Nadal to win it all. But the side courts are way more fun.