Best state in which to incorporate?
Without knowing your home state or nature of your business, it's difficult to give you a direct answer.
With an online business, you could technically register your business with any state. Typically, states such as Wyoming or Delaware are very popular as they are considered the "corporate heavens" along with Nevada. (Though with Nevada's higher set-up and annual fees, it's usually best to incorporate there if you have a physical presence in the state).
Keep in mind, if you are using an address in your home state for the business address and plan on writing-off your business expenses, it may create a nexus in your state, and you may possibly then have to have your business registered in that state as a foreign entity.
It's always highly suggested you consult an accountant with your business plans.