Hall H is the main one. And you're free to go into anyone as long as you're wiling to stay in line for hours.
I wanted to see Green Lantern and no one else from my internship wanted to go with me so I went in by myself. Luckily I met some people in line and made some geek friends.
Anyway, after 3 hours, the WB panel was full. But usually panel after panel (after Harry Potter, people can get in for Green Lantern if there's seats), people tend to leave, giving the people in line a chance to get in.
If you want to stay in Hall H the entire day, you can. For me, I stayed until the end of the 'Let me In' panel and in theory I could have stayed in for Marvel panel. But that would mean I would have to stay in there for another 4 hours and I had to go to dinner with some friends.