I have attended the US Open for several years, although not consistently for some time. You can see quite well in the upper deck of Arthur Ashe. IMO, the best day is the second Tuesday. You are usually guaranteed to see more than one top-ranked player on each of the main courts. You cannot beat the King stadium now for closeness to the action with the upper level removed. I also like the grandstand. I have found it difficult to get into some of the side courts now as the crowds are so much bigger than several years ago prior to the opening of Ashe stadium. In any event, it is still one of my favorite sporting events to attend. I recommend highly watching some at the practice courts where you can get an up close view of just how hard players work, even if they are not top-ten or even 50. Any night session is also a lot of fun. I worked at a clothing booth in King stadium for a couple years and was able to watch each night session. Of course, the later the round the more intense, but the atmosphere on any night is better than any day. Hope you enjoy!
Another fun thing to do if you have time is attend the Ashe junior tournament the weekend prior to the first rounds of the Open. Up and coming juniors are there and many Open players are around practicing. For example, I played in the event twice and both of those times Steffi Graff did as well. She was around 10 and 11 years old and all braces and legs.