What is the best carrier plan for iPhone?

I'm going to the US next month and looking to purchase a 32GB iPhone 5S with a contract. I have never bought any phone or contract in US before so I need your help.

My needs are:

  • 2 year plan, or longer
  • Data: around 2GB per month is fine
  • Carrier must have good coverage

What suggestions do you have?

Thanks in advance.