Right now Montana and S. Dakota are very popular due to the fact they allow you to register with complete anonymity. They have no personal or corporate income taxes and have very low fees to file LLC or Corp. Wyoming was the first state to offer protection for LLC's and its a great state as well. Nevada's asset protection has tons of holes in it and there are more disadvantages to Delaware than advantages unless you are a very large company. Don't forget that just because you incorporate in another state does not exempt you from your own states income tax laws or exempt you from the requirement to register as a foreign entity in your home state. Personally I am registered in Wyoming for an LLC and S. Dakota for a corp. Cost wise, I think Montana is like $89.00 and you can file the paperwork yourself.
If you want to escape having to domesticate a LLC or Corp in another state its pretty easy, get a virtual office for $75 bucks a month with mail forwarding and call forwarding service. Make sure EVERYTHING tied to the company goes through that address and telephone number. Set up your EIN and bank accounts with your virtual address.
If you really want to go crazy, set up a holding company in Panama; then open your corp in one of the US states above and issue all shares to your Panamanian entity. Your holding company bills your corp for "services" "rent" "marketing" and you pay the holding company from the domestic Corp or LLC. Then at the end of the year there is ZERO income ZERO assets ZERO anything available through the US company. The holding company holds your interests and assets and since its never actually conducting business except with companies it owns there is a massive firewall between your cash and any potential liability in the US.
Just my 2 cents.